in response to http://www.harukizaemon.com/2011/04/detecting-unspecified-method-arguments-in-ruby.html

gistfile1.rb

def method(variable=((default=true);nil))
  puts "Value: #{variable.inspect}, default argument - #{!!default}"
end

method nil # => Value: nil, default argument - false
method # => Value: nil, default argument - true

That's an interesting answer but IMHO unnecessarily complex...