How can I tell if my child’s skin rash is from exposure to poison oak?

Streaks, blisters. The allergic rash can cause itching, burning and blistering 1-2 days after contact with poison oak. The blisters may ooze clear fluid and may be different sizes. As they heal, they may look crusty. The rash may also look like pimples.
Contact dermatitis. Poison ivy, oak and sumac all contain the same substance which causes a severe contact skin reaction in most people. The rash will appear from about 24-48 hours after the exposure to the plant oil. It is red, raised, very itchy and often has a seepy discharge or blisters. It will often appear in "streaky" lines on the areas which were exposed. Rash on the face can cause a lot of swelling.