[morzh]

Marty

If you take an unloaded transformer and you DO know where your primary is (finding it by experimentation is not a good idea by any extent of imagination) you simply connect it to variac and bring it up with your voltmeter on at the output winding.

But even if you do NOT know where your primary is it is still OK to try it with the Variac, and here's how: you connect your assumed primary to the Variac output, and have your voltmeter at the assumed secondary; you should know the expected output. Calculate the ratio of the Prim to Sec, then set your variac to say 10V. See if the secondary conforms to the expected ratio or it does not. For instance say your secondary is 6.3V. Set your assumed Primary to 11V by Variac and see 0.6V output. If it is not 0.6V and quite otherwise, say over a 100V - your suspected primary is not really a primary.