The Patch Test is something that you're likely to see on the back of products if you're someone who reads labels. Simply put, it's a fool-proof way to check and see if your skin will turn red, become itchy, or swell due to an ingredient in a beauty product.
Kiehl's is an advocate of the patch test and it's something I recommend to anyone with dry, highly-allergic, or sensitive skin. Apply a small amount of product (about the size of a dime) to your arm (preferably the underside of your forearm), cover it with a Band-Aid, and keep an eye on the area for 24 hours.
I know it seems basic, but it works. You know the old saying about an ounce of prevention, right? If you don't, read more.
"An ounce of prevention is worth a pound of cure."