Recently my battery has been dying in the middle of the day - never did that before. I think I have something running in the background that may be draining it. How do I confirm what apps might be running in the background?

First, you should not let you battery get below 20% on a regular basis. that can damage a lithium battery requiring replacement. Sounds like that could be part of your problem. It is best to keep the battery pluggedin and topped of until you need to run on battery power.

As for apps running in the background, read this from our FAQ: Maximum Battery Life - BlackBerryFAQ

And if you are in a signal fringe area where the device is constantly searching between EDGE and 3G, that is a huge drain.

In options and application management you can check to see what's using the cpu as well as the changing of signal like mentioned. Also having wifi on doesn't help. Something else worth noting is some apps are just badly made, I had an app that was meant to simulate a battery pull, quick pull I think it was called that totally drained my battery over night. After uninstalling the problem went away.