How can you tell if all of your pest snails are dead?

I think I ran out of ramshorns for my assassin snail to eat a few weeks ago. I then got a few more plants from my lfs and I saw 2 more snails. A giant mts and a ramshorn snail a few days later. I killed the mts and the ramshorn is gone. My assassin snail is hunting right now, but I don't know if there's food in there for him to eat. I don't have fish in my tank or anything so there isn't any flake food on the floor to get eaten.