I use a variable resistive load - it must be sized for the higher end of the unit you are testing & then some more for overhead.
Most VOMs(typical max might be 10 amps, BUT DC) will not handle the current you are looking to test for in an AC circuit, a basic 20-25 amp AC panel meter will be a much better choice. An AC voltmeter included in/with your load testing circuit will give a lot of insight as to the actual power delivery capability of your transformer.