Daredevils eat spicy peppers for the rush. Just for the thrill of it. After exposing their mouths to the fire of capsaicin, they usually lose the ability to do anything other than uselessly fan their hands in front of their faces, moan, and cry. Sometimes they curl up into a ball. Those are all universal signs for too hot, and they get the point across, but they're not exactly scientific. We typically turn to the Scoville scale to estimate how hot peppers are. Tabasco sauce sits in the low thousands, while the extreme peppers like the Moruga Scorpion climb up around two million.
There's another way to measure a pepper's hotness--or, rather, the amount of capsaicin it contains--called High Pressure Liquid Chromatography. This is science's way to take on peppers. As pepper site TheChileMan summarizes, in the HPLC test, "A chile solution is placed into the chromatograph machine, and under high pressure, the machine separates the capsaicin from the total volume of liquid and thus calculates the concentration of the capsaicin in parts per million (ppm)."
First a pepper is ground up, and then its powder is placed into a vertical column with a solvent and forced through a packing material like a gel. The test is all about determining how much capsaicin is in the ground up pepper. Because different molecules will adhere to the solvent in different ways, the high pressure test is able to force the different materials into separate layers in the column. And once the capsaicin is separated, scientists can tell exactly how hot a given pepper is.
Though it's still not quite that easy; capsaicin isn't the only heat element in the pepper. There are other compounds, like Dihydrocapsaicin, that bring with them varying degrees of hotness. Each one has to be quantified before the amount of capsaicin present can be multiplied and converted to the Scoville scale. This method has replaced diluting peppers until their hotness can no longer be felt--that was the original way to determine Scovilles--but High Pressure Liquid Chromatography is far more accurate. And it doesn't require tasting peppers over and over again, which is something mouths everywhere should be thankful for.