I try to keep an eye on if I see them eating and drinking. I don't monitor what's in the litter box that closely because with 3 cats, I can't really tell who's doing what if I'm not in there watching them (oh the lack of privacy here). What we do instead is just be aware of how often and how they behave in there. Frequent visits (like three in an hour) or bolting out of the litter box will mean a visit to the vets for UTIs.
I also give them regular kitty massages and we've found growths on two cats this way. I will start at the head and shoulders moving down the body checking behind the hip bones , then the legs and paws. It took awhile to get them use to it but now...
they all love it.
Mostly, I go off of gut feelings. If something doesn't feel right, I will watch that particular cat closer for some sign of something out of the ordinary.