The act of following a questionable male or female to a public restroom to determine his/her actual gender.
"Chris" wore clothes that either sex could wear, had a haircut considered tolerable of both genders, and a voice that's indistinguishable between an angel or a scruffy Southerner. So Joe did the ol' litmus test and followed Chris to the office's bathroom.
The act of covertly sniffing a finger that has been inserted into a woman's vagina while making out for the purpose of determining if it is safe to proceed to oral sex.
I was going to munch her box, but I ran a litmus test, and it came back positive for stank, so I just bagged it and tagged it.