Your birds will need about 3 times the amount of feed in winter and fat can be added by feeding things like wild bird suet, scratch corn and by adding some high-calorie treats, being sure that they have free access to grit and oyster shell.
You would notice adult worms in the droppings. If you have access to a microscope some coop droppings can be mixed with water and examined for eggs. Most tack and feed stores carry piprazine, which is mixed at different dosages depending on the species involved. Ivermectin for horses can be diluted for birds, too. Could order on line, of course. It might be that your birds are at a lean stage as some are when young, getting lots of exercise.
Ya know, in comparison to supermarket chickens, mine are skinny, too. But silkies, Polish, EE's and such weren't bred to be food for humans, so it stands to reason they may not be as plump or muscular as those breeds that ARE used for food. As long as my chickens aren't sickly (yes they've been wormed), and have SOME meat on their bones, I'm not worried. I just make sure they never run out of crumbled or pelleted feed and fresh clean water, I supply fresh scraps as often as possible, and I give them scratch grains w/wild bird food in the winter as a treat. Someone told me that suet is good in the winter as well, so I guess I'll try that once it gets cold here.
I worm in the spring and again in the fall, regardless of whether my birds have worms or not. I am fortunate to be able to run my own fecals, but if I couldn't, I guess I'd be running off to the vet, as sometimes worms can only be detected under a microscope - and just because you don't SEE them, doesn't mean they're not there! In fact, its my understanding that unless they are absolutely LOADED with worms, you won't see them at all! Plus, it helps to know what TYPE of worms they have so that you can use the proper type of wormer that is effective on them.
Oh, and any vet can run a fecal test, whether they work on chickens or not - they don't need to see the animal, just the sample. Poop is poop, regardless of the species it came from, and worms that chickens get are the same type that dogs and other animals get.