The process of checking to see if the girl you are about to get it on with has any STDs
(open sores). While making out, secretly apply some lemon juice to your fingers (you don't actually use a lemon wedge morons) and then rub/insert said fingers into her "warm and fuzzy area". If she begins to scream, cringe, or show pain in any way chances are she has open sores and most likely an STD.
When Jojo was making out with Becky he secretly dabbed his fingers in some lemon juice and gave her the lemon test. When she started to complain about it burning he knew she had the funk!