It’s actually very difficult to know about guinea pigs (unless you are one, I suppose). I once had to take a “sick” GP to a vet…
Vet: What seems to be the problem?
Me: He’s very hot.
Vet: (feeling round GP) Under here?
Vet: That’s what we vets call his testicles.
Me: Ohhh. (pause. Wait for penny to drop) That’s why he’s been chasing Miss Rabbit…?
Vet: Well done. Pay on the way out.