Problem might be that you don't know for sure that the 2 120 volt lines are actually from the two sides of the service. There may be an easier way, but I think you'll need an A.C. voltmeter that goes at least to 250 volts to chec between the two hots to see that they are actually 240 volts apart. They could be just 2 120s from two breakers on the same side of the service.
Tom
