How can you tell the difference between poison ivy, poison oak, and poison sumac?

Look for pictires. The plants are different. Poison oak and summac grow like a bush and poison ivy grows as a vine along the ground or up tree trunks. The reactions that people develop to these are the same. The sap of any of the plants in the rhus genus (poison ivy, oak and sumac are common but also mango, cashew and pistacchio) may cause a rash that can itch and blister 12-48 hours after contact.
Difference in leaf. The poison ivy is leaf is composed of three parts on the same level each the shape of an arrow head. See www.Poisoncontrol.Org on their plant section for a picture. If you flatten the sharp edges and make the leaves rounded like a pin oak you have the poison oak. The sumak is a long multipart fron that resembles more of a fern than a regular plant leaf, its small leaflets are similar ti the ivy.