Taking notes of weeing and drinking and whatnot. You can get diabetic tester strips in most chemists, and you just get some pee on it to get a result.
It's really mostly the Russians dwarfs (primarily Campbells and hybrids) and the Chinese hamsters that are prone.
The gang at home love both of them.
My little robo especially LOVES mealies. And don't forget waxworms! They're scrumptious, apparently.