The speedo cables have to be able to go forward or backward because they are geared to the driveshaft output. The car can certainly go in reverse (unless it's a Tucker!
) try it both ways, one will register forward speed. You need to check it both ways anyway, in case it stopped working first time you backed up! The "traps" for speedo reversal that I've heard of consisted of probes that put a scratch mark along the most significant digit of the speedometer, usually the 10,000 digit. If a "8" was exposed, 0-7 were already scored, so if it was wound back to a "6", a big score line would show through the "6" indicating it had been tampered with.