Rinse new plants under tap water.
Check leaves for snails and snail eggs (clear jelly like clumps).
Put new plants in a quarantine tank for a month and see if anything appears.
You can soak new plants in a bucket of water containing copper for a couple of minutes, and any snails will let go of the plants and sink to the bottom. however, You will still need to check for eggs.
If you put plants in a copper solution, you need to rinse them well afterwards so no copper gets in the aquarium and kills shrimp or snails that you want in the tank.