module_extended.rb

gistfile1.rb

module M
  def foo
    puts "foo!"
  end
end

class C
  def eigen_class
    class << self; self end
  end

  def extended?(mod)
    eigen_class.include?(mod)
  end
end

c = C.new
p c.extended?(M) # => false
c.extend(M)
p c.extended?(M) # => true