If you measure the battery voltage with the machine off then measure it when running, it should be higher when running if the alternator was working.
The voltage across the shunt will only be a few millivolts. The higher the load the higher the voltage.
Don't bother measuring voltage across an ammeter using a standard voltmeter... it will read zero on most low-end voltmeters but in actuality will be a few millivolts.
Think of an ammeter as a fuel flow meter for electricity. An ammeter measures how much electrical energy (not voltage) is passing through a given portion of an electrical circuit... in this case a charging circuit.
Generally the starting/charging circuits operate at fairly high current and therefore by definition they require fat wires. The fatter the wire the more current flow it can accommodate and the thinner the wire the less current flow it can accommodate.
Think of electrical current flow as being water through a hose. A thin plastic garden hose cannot allow as much water to pass through it as a big fat fire hose can. The same thing is true for wires and electricity... a thin wire cannot allow as much electrical current to flow through it as a fat wire. Therefore, by definition, starting/charging circuits must use fat wires (unless you're talking about a lawn tractor that requires relatively little current to do either).
It is not practical to run big fat wires to the back of your ammeter (usually mounted in your front panel) so they do the next best thing... they run smaller wires from the ammeter to a device called a shunt (almost like a fusible link) which is connected between the output of the alternator and the positive battery terminal... one wire to one end of the shunt and the other to the opposite end of the shunt. The theory being that as electrical energy passes through the shunt, there will be a small (very small) voltage drop across the shunt in direct proportion to the amount of current flowing through the shunt. The more current the more voltage drop, and the less current the less voltage drop. An ammeter is nothing more than an expanded volt meter and it is this small voltage drop across the shunt that is being measured by the ammeter.
If your ammeter is not working... but your charging system is... then there are only a few possibilities. First (and most likely) one of the two wires going from the ammeter to the shunt is broken or there is a poor connection either at the ammeter itself or at the shunt. The second possibility is the ammeter itself is defective. There really aren't any other failure points to look for.
