found this:
"Professionals" do it by trial and error too, even while in the middle of a race. In a race car you put the proportioning valve in the cockpit so it can be adjusted on the fly as conditions change.
It's easiest to do it with a friend who can stand, at a safe distance, outside and let you know for sure if that tire that locked up is a front or a rear.
Make sure it's done on dry pavement also. On wet pavement optimal bias is slightly more to the rear than on dry.
If you dial it in on wet pavement, you're going to have too much rear bias on dry pavement.
EDIT: It's also important what technique you use. If you "stab" the brakes you're going to lock up the fronts prematurely because you'll lock them before you get any weight transfer.
Make sure you start your braking firmly but smoothly to let the car's weight transfer to the front, then continue to add pressure until you get a tire to lock briefly and then ease up and come to normal stop. Hopefully your observer outside will be able to tell you which tire locked up.
If you can't find a friend, and all your tires are the same size. On thing you can do is mark all 4 tires with chalk or something, at 12 o'clock on each. Make sure you are going in a straight line. Once you've locked a tire up briefly and come to a normal stop. The tire who's mark isn't in the same position as the rest is the one that locked.
