Observation.

When you're at practice, how are you evaluating yourself?

Maybe you're lucky enough to have mirrors, but are you truly utilizing them or are you just used to looking at your reflection? I wrote a post on mirrors here, and I still see people looking at them while practicing, but without really being able to make adjustments.

Are you looking at other people (mirrors or no) that you deem to be good examples and trying to copy them? It's the same thing with mirrors; if you're looking out of habit but not trying to implement what you're seeing, why bother looking?

I see a lot of people who look at the drum while they play. While there are a few things you can adjust by watching what you're striking, it's not really a great place to look.

There's also an option of closing your eyes and listening to what you (and others) sound like, but it should be done when beneficial, not just because you want to enjoy the music! Is your left hand equal in volume with your right? Are you playing right on the beat or not?

Finally, when you get to watch and not play, what are you looking at? Nothing wrong with just enjoying it, but assuming you're looking at technique, how are you doing it? Are you seeing what people are doing incorrectly and making sure you don't do it yourself?

My point, really, is all about using your time efficiently and effectively. Think about what you're looking at, why you're looking at it, and what benefits you should be getting from it. Don't just practice to go through the motions!