Easy- - - -just connect two fairly large wires, #12 of bigger to the ammeter and use it to power something like a headlight, a heater motor, or some other device that will draw enough current to see the needle deflect. Reversing the direction of flow through the wires will make the needle deflect toward the + or - direction. When current flow stops the needle should return to center. You are aware that dash mounted ammeters are about as accurate and trustworthy as political promises, aren't you?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!