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IIRC, the angle should be between 1.5 and 3 degrees (somebody correct me if I'm wrong).
To measure it, you'll need either a bubble protractor or angle finder.
The shims are usually made of aluminum, and they would be inserted between your springs and the spring perches on the axle.
Here's a link, that I hope won't be pulled: Leaf Spring Axle Shim I just posted it to give a little more info. I know there are lots of places that have these, in fact, probably one of the FTE sponsors.
You will need an angle finder , ussaly 3-5 deg neg (rear end down) from the drive shaft angle, depending if you have traction or ladder bars ... Aluminum shims 2 deg per shim ... Lew