constants.rb

# Module.constants allows you to check all available at point of call constants

module Bar
  AvailableUnderNamespace = :constant
end

class Foo
  include Bar
  p Module.constants.include?(:AvailableUnderNamespace) # => true
end

# Check constants under the Module
Module::FooBar = :constant
p Module.instance_method(:constants).bind(Module).call # => [:FooBar]

# or simpler way
p Module.constants false # => [:FooBar]

# http://bugs.ruby-lang.org/issues/5887

nesting.rb

# http://apidock.com/ruby/Module/nesting/class

module Foo
  module Bar
    p Module.nesting # =>[Foo::Bar, Foo]
  end
end

# http://coderrr.wordpress.com/2009/05/18/dynamically-adding-a-constant-nesting-in-ruby-1-9/