What is the easiest / simplest way to test an ammeter before installing it?
Probably the most important tests are just to make sure it is not internally shorted to its own housing since that could be serious. If you have a VOM (Volt/Ohm meter) then measure the resistance between each of the connections and the housing shell portion of the meter. That should measure infinite resistance (open circuit). Then measure from one electrical connection point to the other and that should measure near zero resistance. If both of those measurements are OK then shake the meter a bit in your hand and see if the needle bounces around and is not stuck. If the meter is a new Fun Projects meter it will not move when you do this but that is OK. If all looks OK then I would mount it and wire it and turn on the headlights. If the wiring is OK then the meter should read somewhere between 8 and 12 amps depending on the size of bulbs you have and whether the headlight switch is in the "bright" or "dim" position. If you have no meter then you could simply wire the meter in series with some automotive headlight bulbs to do the same thing outside of the car. Though an ammeter internal short to its case is rare, make sure your car has a single main fuse in it to protect against such a short causing your wiring some major damage. Hope this helps.
Take a piece of wire and wrap it around a pen for 10 turns making what looks like an open coil.
Place each end on the studs of the ammeter.
Pass a single wire through the loops, and turn on the juice to a load. Inductive load will take the needle in one direction, but not accurately. Reverse the load and it should throw the other direction. This at least shows the movement is not bound or frozen.
As far as accuracy this doesn't do it...but it does show whether it is dead or not. Through the years I have had some meters that refuse to scale and when that happens, I leave the loop across the poles, pass the gen wire through it, and at least it shows me charge or discharge until I can get another meter.