Is the milk in your refrigerator safe to drink, or is it rotten? Open the bottle and take a sniff. If it stinks, it's probably rotten. You don't have to taste it. You don't have to drink a pint of it and see whether you get sick. You know immediately that it's no good. Throw it away and get a new jar of milk.
Stink test is a metaphor for an easy way to determine whether something is right or wrong. If it smells rotten, it probably is. Throw it away and start over. Stink test is a very simple heuristic for determining the validity of a statement or concept. The idea is to trust your intuition; you're probably right.
For example, say your team estimated that it would take one hour to complete a user story. You know that the story is very complex and probably can't be done in less than three days. The one hour estimate smells rotten. It can't be right--it doesn't pass the stink test. Throw it away and start over.
Synonyms include sniff test and scent test. Similar tests are the rice test and the spaghetti test. The spaghetti test metaphor is that you can tell whether your spaghetti is done by throwing it against your kitchen wall; if it sticks, it's done and ready to eat. Do you have a good idea? Are you sure? Lob it out there--does it stick? If it doesn't stick, it's either wrong or it's not ready yet.
Here are some examples: