See Poison Ivy Chemical With Spray:

Before the rash and itching starts, now there's a way to find out if you've run across poison ivy and prevent giving it to others. Chemist Rebecca Braslau of the University of California, Santa Cruz, is very allergic to the oily chemical urushiol on the leaves of poison ivy plants, but her husband, a geologist, isn't. After one too many times of catching the rash from her husband after he'd give her a hug or touch her arm, she started to work on a nontoxic spray that will light up when it comes in contact with urushiol.

Her proof-of-concept spray uses nitroxide chemicals and a fluorescent dye that lights up when in it undergoes a chemical reaction with the poison ivy oil under fluorescent lights. As the spray is undergoing safety trials, currently she uses it on backpacks and clothes. "Ultimately I would love to be able to spray it on my arm, or my husband or my dog, and find out where this stuff is," she told NPR. Then she can use TecNu, a soapy substance with ortho hydroxyls, to get the urushiol oils off the skin.