You take a meterstick, and you hold your hand at the zero, but not touching it. Have someone drop it without telling you when. How many millimeters you have is your reaction. This would also work with inches, but it wouldn't be as accurate.
you can test reaction in a series of ways:
The most common being by taking a long pole and making two marks on it of a certain distance apart such as a metre.
get someone to hold the pole with...
There's alot of ways to test you reaction time the most common one is a stopwatch but most people are confused about when to push the buttons so that's why they get another person to do it for them....