Games : Command block /testfor chat message help, Minecraft

I'm trying to make a building game, with command blocks. I want to make commandblocks testing for chat messages, so if someone writes the word that he is building. I already know how to make the randomiser and all the other commandblock stuff. But the testfor if a player writes for example, Thunder and it's the right word he gets points. But how do i make it testfor that.

Thanks for your help, Jamie