If I ever wonder how some piece of code or a class works or something, I could go write a simple application and test something out. I certainly do not waste my time though. It tends to be much faster just to write a test to answer my question or learn something new. If I want to try something out I just write a test. If it is an internal piece of code I go look at the existing tests, and if it is not I write a temporary one just to try something out.

Unit tests are quite useful for this sort of thing, and since I have keyboard shortcuts mapped to run them they're very fast. If you really want to test a lot, you need to start using them all the time. They become very easy to come up with and write once you practice them enough.