Um. You haven't seen it happening? Since it's winter, I don't spend much time outside. However, I see activity most mornings right after I let them out of the coop. They are in a closed coop (about 10' x 10') with a natural light source. At around 9 a.m. I let them out and that seems to trigger the roosters' "loving feelings," probably because they've got more space. (?) So for the 5-10 minutes that I'm out there, things are happening. Maybe you have a pop-up door that is run by a light sensor or maybe you have a well-fenced run and the chickens can get out as soon as it's light out and so you never see their morning activities??? Maybe keep them closed up until you can let them out in the morning and watch them. I'm grasping at straws, trying to come up with a way for you to see them mating since I suspect they are mating. This is the prime mating period of the year and normal roosters and hens can't help but succumb to nature's promptings.
Not all cases of minimal feather loss on a hen's head are a sign of breeding, but I get a few with that "tell." Also, some hens lose feathers on the backs right in front of their tails from the attention of roosters. Usually the roosters' favorites have this.
If your hens aren't laying, then there is a chance they aren't breeding. But if they're laying, then they look good to the roosters because the hens' combs are red (indicating good hormonal balance, usually).
You can also look at the eggs (when you crack them breakfast) for the germinal disk and the fertilization ring. There are lots of pics on this site showing a fertilized egg versus an unfertilized egg.