How do you find deprecated methods in a rails app?

I'm seeing blog posts where people mention deprecated methods in rails3, for example model.errors.on is deprecated in favor of model.errors[]

My question is, how do you find these deprecated methods? Is there a rake task or something I can run to find them in my project? Do I just need to look them up on a resource like rails3 release notes? Or is ther开发者_开发问答e some other way?

Warnings surely show up in development.log :) BTW, i was asking to myself this question some days ago and i used a very brutal method. Having rails source on my laptop, i searched for all calls to the warn method. Brutal but efficient.