Hi and welcome to the forum
Buying live plants from a tank that does not contain fish helps.
If plants are in pots with gravel, remove pot and gravel then rinse them under the tap.
You can put new plants in a bucket of tap water for a couple of hours and the chlorine/ chloramine in the tap water will kill some of the bacteria on the plants.
If you have concerns about protozoan infections and other harmful disease organisms you can bleach the plants.
Put some bleach (granulated swimming pool chlorine or liquid household bleach) into a bucket and put the plants in it. Let them soak for one and then remove them and rinse off under tap water. Put them in more bleach for another minute then remove and rinse. Then put the plants in a bucket of tap water and double dose with dechlorinator. Stir up and wait an half an hour then tip the water out, rinse and refill. Let them sit in the bucket of water for a few hours and change the water 5 or 6 times during that period (rinsing them each time). Then put the plants in the tank.
If you don't want to use bleach you can quarantine the plants by keeping them in a separate tank/ container for 1 month before moving them into the main display tank.