gistfile1.rb

class Object
  def descendant?(parent)
    parent = parent.class unless parent.is_a?(Class) || parent.is_a?(Module)
    self.class.to_s.index("#{parent}::") == 0
  end
end

module MyModule
  module AnotherModule
    class MyClass
    end
    
    class AnotherClass
    end
  end
end

module MyAnotherModule
end

m = MyModule::AnotherModule::MyClass.new
p m.descendant?(MyModule)
p m.descendant?(MyModule::AnotherModule)
p m.descendant?(MyModule::AnotherModule::MyClass)
p m.descendant?(m)

a = MyModule::AnotherModule::AnotherClass.new
p m.descendant?(a)
p m.descendant?(MyAnotherModule)