I would generate a non-symmetrical waveform like maybe a sawtooth and apply it. You can determine the phase easily (that might be important lol) as well as something of the turns ratio.

After that switch to a square wave to get the turns ratio more accurately. Rock the frequency to where it is the most efficient (puts out the most). Some transformers will respond well enough with the sawtooth, but the square wave is alot easier to measure. Also, if you need to determine the current capability of each winding, that is also easier to do with a square wave.

You can determine this all without a scope, but a scope makes it all alot easier using the sawtooth. With the square wave you would have to try to boost and buck each winding to see which way is "up".

I would suggest checking the current capability of each secondary if the voltages are close because alot of times there is not a tapped secondary, there are two secondaries that are connected together externaly.

All worst case scenario of course. If the thing only has five wires or something it is a piece of cake. Too easy to even ask about.