At my vet, I drop in with a cat and get a weight, no charge, no appointment. I usually go in the mornings when they are doing surgeries, so there are no appointments, the exam rooms are just sitting there empty.

I know it is a nuisance to get the cat in the carrier and take him over, howling all the way if he is like mine. But it is an idea, and I've done it a few times over the years when I was wondering about stuff.