Is it an AC/DC set? That's probably one way to tell. Look for a mains transformer (don't confuse it with the output one - that's connected to the speaker).
If it lacks one, it's an AC/DC set -- the tube filaments are all connected in series.
The usual American combination of these (in the 1960s-70s at least) was usually a 12AV6, 12BE6, 12BA6, 35W4 and 50C5 - these are the radios I'm most familiar with and have a few of.
Add up the first numbers in the tube codes and you'll get the line voltage the set is supposed to work on - 121 volts in this case.
It was called an All-American 5 and the idea was to eliminate the high cost of the transformer.
The ones designed to work on 220 volts would also have a dropper resistor (big fat ceramic thing) in series with the heater string.
Warning: when working on one of those, use an isolation transformer!!!!
Usually you will find the big electrolytic mains filter cap to be leaky and all these paper and bakelite-encased "black beauties" capacitors to be shot. Replace them all with comparable values.
Then check the resistors and the tube sockets. On rare occasions you'll find a bad tube.
