Kendra Little discusses how to test cumulative updates:
Install the cumulative update into a non-production environment (prod test or staging) and run application activity against it.
Ideally, you want this to be representative activity for your real, production applications, run from multiple application servers. You also want the hardware and storage to be as similar as possible.
If that’s not possible for you to set up, you need some sort of tool to generate concurrent activity. There are a lot of options — Michael Swart gives a great run down here.
This is some very good advice, and boils down to “test everything that you do in production.” It’s easy to forget some of these things, though, so I’m glad Kendra enumerated them.