How do you know if a rash was caused by poison oak?

See below: The rash of poison oak often has a linear configuration reflecting the points of contact of the broken stem of the plant with the skin.

Poison Oak Rash: Typically an itching, red rash with bumps or blisters in a linear pattern or wherever the oil touched. The rash does not spread by the fluid from the blisters. Therefore, once a person has washed the oil completely off the skin, the rash is usually not contagious.