method_defined_within.rb

def method_defined_within?(name, klass, superklass = klass.superclass) # :nodoc:
  if klass.method_defined?(name) || klass.private_method_defined?(name)
    if superklass.method_defined?(name) || superklass.private_method_defined?(name)
      klass.instance_method(name).owner != superklass.instance_method(name).owner
    else
      true
    end
  else
    false
  end
end

class Foo
  def hi; "hi"; end
end
class Bar < Foo
end

p method_defined_within?(:hi, Foo) # => true
p method_defined_within?(:hi, Bar) # => false
p method_defined_within?(:hi, Class.new(Bar)) # => false

module M
  def say; "say what?"; end
end
class A
  include M
end

module N
end
class Beta < A
  include N
end

p method_defined_within?(:say, A) # => true
p method_defined_within?(:say, A, M) # => false

p method_defined_within?(:say, Beta) # => false
p method_defined_within?(:say, Beta, N) # => true (!!!)