Its almost certain its an amp gauge. However, if you want to check, go get an ohmmeter and check the resistance. If it has a high resistance, its a voltmeter. If it has a low resistance, its an ammeter. If you hook up an ammeter wrong, it will just read backwards - so if you do hook it up wrong just switch the terminals. If it ends up being a voltmeter, MOMENTARILY touch the leads to a 12v battery. If you touch the leads and it pegs backwards, its hooked up wrong. This procedure will not harm a voltmeter as long as you do it momentarily - because there is enough resistance in the gauge so that not much current will pass through it.
