A cat can look completely healthy and have a urinary tract infection - or worse. My cat Martell had crystals in his bladder and the only sign he exhibited was occasional wetting outside the litter box. He was diagnosed by having a urine sample tested by the vet. I never would have suspected that anything was wrong with him otherwise, as cats are very good at hiding pain and he seemed healthy as could be.
The danger with crystals in the bladder is that they can block the urinary tract if left untreated, and this can actually be a fatal condition. There is more information via Cornell Feline Health Center here
Be sure to watch the cats to see if one goes to the litter box frequently, seems to strain or cry in the litter box, or goes to the box only to pass a very small amount of urine (or sometimes even none at all). In some cases you may also be able to detect blood in the urine, but not always.
I get my cats to the vet at the first sign of urinating outside the litter box. A cat's instinct is to bury its waste to avoid detection by predators, so urinating outside the box is often a sign that something is wrong.
Your vet could give you instructions for collecting a clean urine sample from each cat which they may be able to test without even taking your cat in for an exam.
My cats have all been spayed or neutered before being introduced to my household, so I can't offer any suggestions about that. It might be worth a call to your vet to ask their opinion on the matter. My vet has always been willing to offer advice over the phone, and to tell me whether she believes the cat should be examined or not.