Radar gun is the only way to do it easily. However, I had recorded myself on a digital camera. I took the sound file and basically "looked" at the peaks of the sound of the tip hitting the cue ball and then the cueball hitting the rack. I could see the exact time between these peaks. I measured the distance and calculated and it was quite accurate.
Anyway, some info you should know about a radar gun:
1) I have the "Sports Radar" gun pictured above. It works well. It will not give you more accuracy than 1mph, but that should be fine.
2) Flourescent lights mess with radar guns. You will have a much easier time using one near incandescent bulbs, unless the flourescents are far away from the table.
3) Different radar guns have different "target aquisition speeds". The Sports Radar on is ok. If you really want a nice one and have the money to spend, the Stalker brand guns are far superior (but like $800 minimum)
4) You do not need someone else to operate the gun. The gun has a trigger mode, and a "continuous" mode. Also, it comes with a base to mount it to a tripod. You can get one at a camera store for under $50. Then you just set it on continuous and it will record each new break speed. obviously it will erase the old one, so you have to walk over and look at what the speed is.
5) They make a display device for this and other radar guns. Basically, it attaches to the gun and gives a big display, say 5" tall numbers on a led screen. You could point the screen towards where you stand so you could immediately see the speed without having to walk over to the gun.
6) For accurate readings, you need to be shooting the cueball directly at the gun. It cannot be to the side or anything. Also, you don't want the gun up high, pointing down at an angle. You need to have it just higher than the rack of balls, so that you can "aim" it at a point on the cloth in front of the rack. You always see those guys in the booth standing with the gun like 3 feet above the table. This will reduce the speed reading.
If you get youself set up with something, there are some fun things to try that might even be more useful than seeing what your max speed is:
1) Try to pick a speed in the middle of your range and see how repeatably you can hit that exact speed.
2) See what the max speed is without letting the cue ball touch a rail.
3) Find out the softest speed you can hit and still drive 4 balls to a rail.
4) Have a friend suggest a speed to you and see if you can hit it.
I think being able to be accurate about what speed you are hitting can greatly help your ability to consistently pocket balls on fussy tables.
Just my $0.02
KMRUNOUT