If you think of "fitness buffs" as being a special clique, then noticing other potential members of the clique and feeling like you belong to something bigger than yourself can reinforce your "fitness buff" identity.

That's not exactly what I was saying, although your point here is correct too I suspect. My point is that fitness buffs have a tendency to look at other peoples' bodies just as car buffs have a tendency to look at other peoples' cars and gun buffs have a tendency to look at other peoples' guns.

However, this technique can be very counterproductive. Women in the US and elsewhere are often socialized to compete with each other on the basis of appearance already, so noticing fitter women is already something that we do with not necessarily very positive results.

Well that's a different issue. But I do think there's more of a problem with people putting not enough mental energy into fitness than too much.

By the way, the idea that you can tell how fit or healthy someone is just by looking at them isn't correct.

You can make a decent guess about a person's level of fitness based on their appearance. To be sure it will not be perfect. Besides which, the point is not to be a perfect judge of fitness -- the point is to find ways to tweak one's goals in order to have sufficient motivation to go to the gym regularly.