How do I tell if a cat is nursing?

I noticed a pregnant ferrel cat in my parking lot.She disappeared when I tried to trap her for vet check.She reappeared a wk after delivery.I havent spotted the kittens yet but I have been feeding her regularly. I want to get het spayed now.How do I tell if she is nursing.I havent seen the kittens 4 wks after delivery.How long do I wait.Its winter now and I dont want to wait for too long to get her spayed which I cant if she is lactating.Please advise.