Long story short the different metabolites in most cases. Your body breaks down what it takes in, and eventually pushes it out one way or another, through sweat, pee, poops. Things like THC for example will be broken down in a specific way that can be detected in urine tests by reacting to different chemical agents.
Waste products from your body processing the drug can be found in your urine.
For a real eli5 comparison, if you look super close at your poop you might see remnants of what you ate. Corn is a famous one. That’s the waste product from your body processing the food. You can tell, to a certain extent, with just your eye what you’ve eaten.
Drug tests are more sophisticated and look for specific chemicals that are left over when your body processes drugs.
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