I've been flying RC electric for years, and the piece of equipment that I lack is an ammeter with sufficient range!
You'll want a good* digital VOM. You want the voltmeter to make sure that you've got voltage where you're supposed to have voltage, and you want to be able to measure continuity to make sure that connectors are connected and wires are unbroken.
I really need to get or make an ammeter, though.
* You need good as in 'rugged', not necessarily good as in "the most accurate thing in the world". I use a Fluke 77 which is both, but that comes from my professional life, not because I needed it for electric flying.