diff --git a/etc/completion.bash b/etc/completion.bash
index d8731c65bff7..9263289b5b4e 100644
--- a/etc/completion.bash
+++ b/etc/completion.bash
@@ -66,7 +66,7 @@ _crystal()
                 _crystal_compgen_options "${opts}" "${cur}"
             else
                 if [[ "${prev}" == "tool" ]] ; then
-                    local subcommands="context dependencies format hierarchy implementations types"
+                    local subcommands="context dependencies flags format hierarchy implementations types"
                     _crystal_compgen_options "${subcommands}" "${cur}"
                 else
                     _crystal_compgen_sources "${cur}"
diff --git a/etc/completion.fish b/etc/completion.fish
index 150dd37108d8..64fc6a97b45a 100644
--- a/etc/completion.fish
+++ b/etc/completion.fish
@@ -1,5 +1,5 @@
 set -l crystal_commands init build clear_cache docs env eval i interactive play run spec tool help version
-set -l tool_subcommands context expand format hierarchy implementations types
+set -l tool_subcommands context expand flags format hierarchy implementations types
 
 complete -c crystal -s h -l help -d "Show help" -x
 
@@ -173,6 +173,8 @@ complete -c crystal -n "__fish_seen_subcommand_from expand" -s p -l progress -d
 complete -c crystal -n "__fish_seen_subcommand_from expand" -s t -l time -d "Enable execution time output"
 complete -c crystal -n "__fish_seen_subcommand_from expand" -l stdin-filename -d "Source file name to be read from STDIN"
 
+complete -c crystal -n "__fish_seen_subcommand_from tool; and not __fish_seen_subcommand_from $tool_subcommands" -a "flags" -d "print all macro 'flag?' values" -x
+
 complete -c crystal -n "__fish_seen_subcommand_from tool; and not __fish_seen_subcommand_from $tool_subcommands" -a "format" -d "format project, directories and/or files" -x
 complete -c crystal -n "__fish_seen_subcommand_from format" -l check -d "Checks that formatting code produces no changes"
 complete -c crystal -n "__fish_seen_subcommand_from format" -s i -l include -d "Include path"
diff --git a/etc/completion.zsh b/etc/completion.zsh
index bf57d80208df..ffa12798ca18 100644
--- a/etc/completion.zsh
+++ b/etc/completion.zsh
@@ -165,6 +165,7 @@ _crystal-tool() {
         "context:show context for given location"
         "dependencies:show tree of required source files"
         "expand:show macro expansion for given location"
+        "flags:print all macro 'flag?' values"
         "format:format project, directories and/or files"
         "hierarchy:show type hierarchy"
         "implementations:show implementations for given call in location"
@@ -211,6 +212,12 @@ _crystal-tool() {
             $cursor_args
         ;;
 
+        (flags)
+          _arguments \
+            $programfile \
+            $help_args
+        ;;
+
         (format)
           _arguments \
             $programfile \
diff --git a/man/crystal.1 b/man/crystal.1
index 3a2a3d4a1b2a..e137959dd5ad 100644
--- a/man/crystal.1
+++ b/man/crystal.1
@@ -367,7 +367,7 @@ Disable colored output.
 .Op --
 .Op arguments
 .Pp
-Run a tool. The available tools are: context, dependencies, format, hierarchy, implementations, and types.
+Run a tool. The available tools are: context, dependencies, flags, format, hierarchy, implementations, and types.
 .Pp
 Tools:
 .Bl -tag -offset indent
@@ -396,6 +396,8 @@ Show skipped and heads of filtered paths
 .El
 .It Cm expand
 Show macro expansion for given location.
+.It Cm flags
+Print all macro 'flag?' values
 .It Cm format
 Format project, directories and/or files with the coding style used in the standard library. You can use the
 .Fl -check
diff --git a/spec/compiler/crystal/tools/flags_spec.cr b/spec/compiler/crystal/tools/flags_spec.cr
new file mode 100644
index 000000000000..c295439e0547
--- /dev/null
+++ b/spec/compiler/crystal/tools/flags_spec.cr
@@ -0,0 +1,44 @@
+require "../../../spec_helper"
+include Crystal
+
+private def parse_flags(source)
+  Crystal::Command::FlagsVisitor.new.tap do |visitor|
+    Parser.parse(source).accept(visitor)
+  end
+end
+
+describe Crystal::Command::FlagsVisitor do
+  it "different flags" do
+    visitor = parse_flags <<-CRYSTAL
+      {%
+        flag?(:foo)
+        flag?("bar")
+        flag?(1)
+        flag?(true)
+      %}
+      CRYSTAL
+    visitor.flag_names.should eq %w[1 bar foo true]
+  end
+
+  it "unique flags" do
+    visitor = parse_flags <<-CRYSTAL
+      {%
+        flag?(:foo)
+        flag?("foo")
+        flag?(:foo)
+      %}
+      CRYSTAL
+    visitor.flag_names.should eq %w[foo]
+  end
+
+  it "only macro" do
+    visitor = parse_flags <<-CRYSTAL
+      flag?(:flag)
+      f.flag?(:foo)
+      F.flag?(:bar)
+      {% f.flag?(:baz) %}
+      {% f.flag?(:qux, other: true) %}
+      CRYSTAL
+    visitor.flag_names.should eq %w[]
+  end
+end
diff --git a/src/compiler/crystal/command.cr b/src/compiler/crystal/command.cr
index 97bf46feb663..651bc62900ef 100644
--- a/src/compiler/crystal/command.cr
+++ b/src/compiler/crystal/command.cr
@@ -41,6 +41,7 @@ class Crystal::Command
     Tool:
         context                  show context for given location
         expand                   show macro expansion for given location
+        flags                    print all macro `flag?` values
         format                   format project, directories and/or files
         hierarchy                show type hierarchy
         dependencies             show file dependency tree
@@ -177,6 +178,9 @@ class Crystal::Command
     when "format".starts_with?(tool)
       options.shift
       format
+    when "flags" == tool
+      options.shift
+      flags
     when "expand".starts_with?(tool)
       options.shift
       expand
diff --git a/src/compiler/crystal/tools/flags.cr b/src/compiler/crystal/tools/flags.cr
new file mode 100644
index 000000000000..17bb15007021
--- /dev/null
+++ b/src/compiler/crystal/tools/flags.cr
@@ -0,0 +1,104 @@
+require "colorize"
+require "../syntax/ast"
+
+class Crystal::Command
+  private def flags
+    OptionParser.parse(@options) do |opts|
+      opts.banner = "Usage: crystal tool flags [path...]\n\nOptions:"
+
+      opts.on("-h", "--help", "Show this message") do
+        puts opts
+        exit
+      end
+
+      opts.on("--no-color", "Disable colored output") do
+        @color = false
+      end
+    end
+
+    visitor = FlagsVisitor.new
+    find_sources(options) do |source|
+      Parser.parse(source.code).accept(visitor)
+    end
+    visitor.flag_names.each do |flag|
+      puts flag
+    end
+  end
+
+  def find_sources(
+    paths : Array(String),
+    stdin : IO = STDIN,
+    & : Compiler::Source ->
+  ) : Nil
+    stdin_content = nil
+    paths.each do |path|
+      if path == "-"
+        stdin_content ||= stdin.gets_to_end
+        yield Compiler::Source.new(path, stdin_content)
+      elsif File.file?(path)
+        yield Compiler::Source.new(path, File.read(path))
+      elsif Dir.exists?(path)
+        Dir.glob(::Path[path].to_posix.join("**/*.cr")) do |dir_path|
+          if File.file?(dir_path)
+            yield Compiler::Source.new(path, File.read(dir_path))
+          end
+        end
+      else
+        Crystal.error "file or directory does not exist: #{path}", @color, leading_error: false
+      end
+    end
+  end
+
+  class FlagsVisitor < Visitor
+    @in_macro_expression = false
+
+    getter all_flags = [] of ASTNode
+
+    def initialize(@flag_name : String = "flag?")
+    end
+
+    def flag_names
+      all_flags.map { |flag| string_flag(flag) }.uniq!.sort!
+    end
+
+    private def string_flag(node)
+      case node
+      when StringLiteral, SymbolLiteral
+        node.value
+      else
+        node.to_s
+      end
+    end
+
+    def visit(node)
+      true
+    end
+
+    def visit(node : Crystal::MacroExpression | Crystal::MacroIf | Crystal::MacroFor)
+      @in_macro_expression = true
+
+      true
+    end
+
+    def end_visit(node : Crystal::MacroExpression | Crystal::MacroIf | Crystal::MacroFor)
+      @in_macro_expression = false
+    end
+
+    def visit(node : Crystal::Call)
+      check_call(node)
+      true
+    end
+
+    private def check_call(node)
+      return unless @in_macro_expression
+      return unless node.name == @flag_name
+      return unless node.obj.nil? && node.block.nil? && node.named_args.nil?
+
+      args = node.args
+      return unless args.size == 1
+      arg = args[0]
+
+      all_flags << arg
+    end
+  end
+end