Yes: shove a mirror under the person's nose and see if it fogs up. If it does, there's definitely serotonin present in the brain. Serotonin is a neurotransmitter; if there wasn't any of it in the...
This is a good question, and one important to all scientists.
The best, single, answer would be - control your variables.Let's say I've synthesized a compound in my lab and I want to know what the...