Actually, this is not how it is done scientifically. They actually make detailed profiles of brain state, develop causal and analytical models, using scanning and computer technology. All this with input from the person they are scanning, of course. This has allowed a much greater understanding of what actually are the mechanisms and processes involved in emotion, rather than fuzzy semantics, to the extent that they could actually tell you what you what you are feeling at the moment if they hooked you up to a brain scanner.
Of course, the same technology is behind some of the experimental brain scanning "truth machines". Even though people lie, some very convincingly, it is relatively easy to identify a lie with brain imaging techniques. Unlike the polygraph, which sometimes fails because it is only measuring secondary effects, brain scanning technologies go straight to the source and the brain is a veritable "red light, green light" when people are lying.