ok, ive found this very usefull. this is how u do it.
Measure exactly how far your pitching from.
get some one to time how fast it takes you to throw the ball to the backcatcher/fence. (stopwatch)
take the numbers and divide distance by time.
say some one throws the ball from 65feet in .70 seconds. divide 65 into .70 and you should get the speed. 65/.70= 92mph
hope it helps, if theres any problems just tell me and ill fix that.