Basically, look at what you know about the world and other people. If what you are telling yourself contradicts with that, one or both of those ideas is irrational.

Example:
If you think people should all be treated well, its irrational to think you shouldn't treat yourself well. If people should be treated well and you are a person, you should treat yourself well, too.

Rationality is a bigger idea than this, this is just the example. I was having a tough time explaining it, my mind is cloudy.