A volt gauge will tell you the charge state of the electrical system by just turning on the key, Amp gauge only works if its running or how much current draw there is.
What is a Amp gauge?
An amp gauge measures the actual amount of electrical current flowing into or out of the battery. The amp gauge needle will move to the (-) when current is flowing out of the battery, such as when the generator output cannot keep up with the electrical load.
What does a volt gauge measure?
This type of gauge tells you how much and in which direction the electricity is flowing. This means the gauge will measure voltage across a ground and a positive wire. New cars use voltmeters, which are preferred by most street rodders. Most alternators typically put out between 13.8 and 14.5 volts while charging.