Not really any way to know for sure. Their appetites will increase and of course their bellies will begin to fill out. This next bit might sound horrible. I'm sorry if it causes upset - your vet would probably spay them even if they are pregnant. Obviously it is a bigger procedure and, also obviously, any kittens wouldn't make it : ( . Sorry to bring it up. Ignore me totally if you think I'm speaking out of place. I would! I'll be honest, its not something I could do, but I wouldn't judge anyone who did it. You could even tell the vet your suspicions and ask not to be told if there were kittens present. Again, I'm sorry to bring it up. On a more positive note - cats are seasonal breeders and much less likely to become pregnant during the Winter months x