LAST EDITED ON Apr-27-09 AT 08:25AM (MST)[p]A scale. Weigh it.
Not sure what else you are asking. Something in the range of 400 grains is what most prefer and for good reason.
Or are you talking about arrow spine? For that check the arrow selection chart for your particular brand. That will tell you which shaft you should use given your poundage, point weight, and shaft length. This is key. No archery set up will shoot well without a properly spined arrow.