Best way to test gauges???

For the purposes of testing a gauge, a 6V battery will work fine. If you need to verify accuracy, it's a little more complicated.

If you just want to veryify that it works, connect it across a 6V lantern battery. You don't need something that will supply lots of current. Make sure the gauge operates smoothly thru the full sweep. Don't leave the gauge connected longer than you have to, but it will survive a few minutes of this with no problems. You may notice that the gauge reacts somewhat slowly compared to how it operates in the car. This is normal and I'd be happy to discuss why in PM's if you wish.