You can catch a lot of the poop and pee by putting litter boxes where they go to the potty the most -- usually under the hay rack or in a corner of the cage.
You can put a towel or pad on your lap, and then pay attention to what they're telling you. Usually they'll squirm, or pull on the pad, or nibble you when they're ready to get back in the cage. If you hold them after that, you'll likely get peed on.