Are there litter boxes in there? Does anyone use one? If so, take one fecal sample into the vet to have them take a look for internal parasites. If there's no sign of them in there, I'd assume none of them have them and certainly during this season I wouldn't worry about fleas or ticks either.
If there are internal parasites in the fecal sample, then I would assume all of them have the problem. I don't think there's a topical that treats only internal parasites (meaning something that would be cheaper). Of course - depending upon the type of internal parasite (if any) - how do you feel about giving them meds?
If they have tape worm, it's one pill. One dose and that's it (if I'm remembering correctly). If it's round worm, then it depends. You can try the same pill - but some round worm is resistant. Most vets prescribe a liquid that has to be given once or twice a day for 10 days. The issue with round worm is that on top of that you have to treat again after three weeks, because the poison only kills the adult, so you have to wait for the eggs (if there are adults in there, there are eggs) to mature, then kill them.
But to treat internal parasites only would be cheaper... but more difficult. !!! (The topical is obviously very convenient compared to the alternatve). If they have worms - they really should be treated.
That said.... if they're putting on winter weight and you can tell they are? Then it isn't a bad problem and probably wouldn't hurt to wait until Spring. Internal parasites are really the biggest problem for kittens. Not that anyone is OK with their cats having worms...
...and I was talking about treating the tom with the same meds you'd use on the garage cats. But there is a big reason to get him neutered - you do NOT want him to start spraying around your house - especially not in your garage. Him being older, if he starts, it's something he may not stop even once neutered. And that is a nightmare, really. Un-neutered male cat pee is SO strong smelling and a nightmare to get rid of. Couple of (potential) nightmares there.... (He'll also get a lot nicer once he's neutered!