you can toss in some veggies and that will get the snails to move on over for them. You can then take out the veggie with the snails on it, kill them, and then put in some more veggies to see if there are any remaining. Do keep in mind that there may be snail eggs on the plants and you will have to wait until they hatch before you put them in your tank. Repeat once or twice a week for 2 weeks and once you come out clean then they are ready for your tank.
I personally don't mind some of the snails, they've kept my tank free of hair algae that I have been having trouble with and as long as they keep that up I'll keep them in my tank.