How to determine correct voltage input on a multivoltage power-transformer?
First method, the safest one: check it on schematics, manual or photos in net.

Second method (commentators were close but not exactly): measure the resistance between power plug pins, NOT every single primary coil, in every position of selector. Measuring resistance or voltage on every single coil gives us no meaningful information if we don't know how selector connects coils (look at method one).

In nearly every old radio which I know (and other equipment) voltage selector doesn't switch between single coils. Coils are connected in series (one winding with several taps) and selector just connect one line of main supply to one of these taps. For example if we have a 220V/110V selector the transformer should have two identical primary coils (both rated for 110V), at "110V" mains is connected to only one coil (second one is left floating), and at "220V" coils are connected in series and powered from the start of first coil to the end of second coil. I've attached a part of schematic of old Swedish AGA radio - there is one winding with five taps and switch to select them.