My son recently had an allergic reaction to sunscreen, among other things that I was unaware of. I decided it was time for him to be allergy tested. I had a lot of misconceptions going into the testing. For starters, I thought that the doctor would take some kind of long needle things and make long scratches on my sons back. I also thought he might have to be strapped down or be on some kind of fancy machinery or equipment. I was pleased to find none of that to be the case.
We arrived and the doctor had my 3 year old son lay down on his stomach and then they took 3 sets of presses (for lack of a better word) that each had 8 different sharp feet that have known allergens on them. 
The nurse and I held Dylan down while the other nurse made the quadrants on his back and pressed each set onto his skin. They were 3 brief but painful presses.
Then we waited... 20 long minutes but I knew we had discovered something. There were 3 allergens that were causing inflammation on his back: Dog, Cat, and Grass. The resulting recommendation was to give him 24 hour zyrtec daily for 2 weeks and then have a follow up appointment. And then it was ducky's turn...
You might also be interested in reading:
Follow up Toddler Using Nasal Spray for Allergies
My Toddler Is Allergic To Sunscreen