unfortunatly, there is no real way of telling the difference just by looking at them. perhaps with a pair of vernier calipers, then you would have to know what to look for. im pretty sure that for the imperial stuff, the pitch is acually the number of teeth per inch. i remember long ago when the tamiya mini was first released, the biggest pinion you could get for it was a tamiya 20 tooth. it had preset motor mounts. guys at racing used to squeeze imperial 21's into them (they would physically fit!) but due to the difference in pitch, the spur gears wouldnt last long.