A test done by men during foreplay, to see if a girl is clean enough to go down on. At some point after manual contact has occurred, you get your hand back into range of your nose and sniff without her noticing, easily done behind her back while kissing the side of her neck.

When you eat dirt for the first time and leave your family to build a shack in the woods

A method (usually utilized only by the male population) to determine if an article of clothing is wearable. To administer the test, simply hold the piece of clothing to your nose and take a whiff. If the item passes the smell test (or smells relatively clean), it is generally assumed to be safe to wear. If the article smells like a gym locker, then it will usually be tossed aside and the wearer will look for another piece of clothing to wear instead.

When you have no more clean clothes and the only clothes that you have are on the floor. So you go and pick them up and give them a good sniff, if they don't stink you wear it.

the test mom's give their teens to check for cigarettes, pot or alcohol.

Contrary to the clothes and snatch smell test, the post-poop smell test is used to determine whether or not the pooper's hands need cleansing. Positive results = fingers smell like poop = hand has come in contact with butthole = hand-washing necessary. Negative results = finger smell does not resemble feces = successful wipe = hand-washing deemed frivolous