It's helpful if you have a variac (NOT a lamp-dimmer triac) so you can
run the voltage up slowly. An alternative that works nicely is an
incandescent light bulb in series with the 120V supply. If the series
lamp, 100 watts or so, lights very dimly or not visibly at all, with
nearly all the voltage across the transformer winding, you've almost
certainly got it connected to the primary. The light bulb will keep
you from blowing breakers or fuses. If the transformer has an internal
short or you're hooked to the secondary (or if you short the secondary
or put a heavy load on it), the lamp will light much more brightly.
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To check the windings of any transformer I always use another mains
transformer with a low secondary winding output of say 12 or 6 volts. I
connect the low voltage output to one of the windings of the 'mystery'
transformer and with a DVM check the voltages on the other windings .This
provides the winding ratios ,from which the actual operating voltages can be
derived ,with additional information from winding resistances.

To check the current ratings of a secondary winding I increase the load on
that winding until the output voltage is 95% of the 'no load' voltage . The
corresponding current is the safe maximum current to flow through the
winding .
Depending on the transformer's output voltage ,I use car bulbs or a lenght
of fencing wire (iron) in a bucket with water for low voltages , light
bulbs (parallel and/or in series ) for voltages up to 500 V or even higher
(be careful).

Frank GM0CSZ / KN6WH