They use a device called a Perry Gordan Measurement Device. Named after the two people who invented it. It looks a little like a bowling bag with a fleshlight stuck in it. A volunteer is provided with the erotic material of their choosing to help them achieve erect state and then the subject inserts themself into the fleshlight part and until they ejaculate within the machine. At which point sensitive photo voltaics and micro lasers are used to measure the speed with military accuracy. Or not. Cheers!