gistfile1.rb

class Object
  def expect m
    (class << self; self; end).class_eval do
      define_method("#{m}_") do
        $expected = true
        send("__#{m}__")
      end
      
      alias :"__#{m}__" :"#{m}"
      alias :"#{m}"     :"#{m}_"
    end
  end
end

class Foo
end

$expected = false

@foo = Foo.new
@foo.expect(:inspect)

@foo.inspect

puts "inspect called on @foo!" if $expected