How do I test for commands in other command blocks?

So I had a crazy idea, which is a surprise that I will later be releasing, but I needed some help with /testforblock . Basically, I wanted to find out how to use /testforblock to see if there is a certain command in a command block. I know it's possible, because I've seen people like SethBling do it. Also, I know how to set up a testfor system already, so I don't need help with that. I'm guessing this is similar to how you check for items in chests(I don't know how to do that, either). If anyone knows, please tell me. Thanks!