Work what out? The speeds?
Finally, we've got everything sorted out - so how fast do you actually bowl? Naturally, the best thing possible to use would be a radar gun - however they're not generally widely available to clubs and schools - but there is a simple way that needs just a stopwatch and a calculator (or maths geek) to provide approximate results. Start the stopwatch when the ball is released, and then stop it again when either the batsman hits (or misses) it, then divide 45 (or 72) by your answer to get your answer in miles per hour (or kilometres). For example, a delivery timed at 1 second has travelled at about 45mph (72kph). To break the magical 100mph (160kph) barrier, the clock must register at 0.45 seconds. Once you get used to working the timer, you will get surprisingly consistent results. I've found that standing at the back of the net, either directly behind or just to the side, gets the best results.
Stolen from CW, I haven't seen the article in ages, but found this on the forum in another thread.