[morzh]

Logically thinking you want your test voltage upon plug-in to be on the order of what you expect from the lowest of the voltages you have from your secondaries.

For example if you have your 110V/6.3V+12V+2V transformer you want to start from about 2V to 5V and then look at the rest. Be careful though - your 110V will produce 260V if you put 5V to 2V winding and touch the 110V with both hands you will be unpleasantly surprised. Just hook up the meter and keep measuring. Once you see that the outputs do not produce much voltage it is likely you found your correct primary (as 5V divided by about 20 will be in 200mV range) and you can start gradually bringing your voltage up until it is 110V and you see the respective outputs. They will be somewhat higher but plausible. Maybe 7V instead of 6.3.