How do you figure axel weight on a semi truck?

Well, you either go to a CAT scale, which weighs each groups of axles separately, or, if you're on a scale which only shows gross weight, then you axle out. You drive onto the scale and stop with only your steer axle on the scale. Then you write down that weight, and pull forward until your drive and steer axles are both on the scale. Then you write down that weight, and subtract the weight of your steer axle to get the weight on your drive axles. Then you pull forward until all axles are on the scale. This will be your gross weight, and you subtract the weight you got when you had the drive and steer axles on the scale in order to get the weight on your trailer axles.