Modifying to 220V operation. How to make sure?

I like the idea of first removing the tubes for safety's sake, seems like a very good idea. When you power up, try measuring the filament voltage at a tube socket. If you get twice the reading that you expect then you're probably applying the 240V to a unit actually wired for 120V. If you get the correct filament voltage (may be slightly high because there's no filaments loading the transformer output) then it appears you're already setup for 240V. Cathode & anode voltages could vary significantly, especially with no tubes in the circuit; it's hard to say what you should expect to see there.

Even with the tubes removed you still have the potential to do some serious damage. If you do it wrong and plug it into 220, you could have twice the normal DC voltage in the amp. That means twice the normal DC voltage across the filter caps. In my amp that would mean going from a B+ voltage of about 500 volts to 1000 volts. This is sure to be more than the maximum rating and since electrolytic capacitors tend to explode when they have too much voltage across them, you may have a real mess. No telling what else would be destroyed. Perhaps you.

Marakanetz, you seem to know just enough to be dangerous. I mean that in the nicest way. I admire your inquisitive nature and your desire to improve your system. But you are going into areas that will result in disaster unless you know what you are doing. Perhaps you could get some textbooks on basic electronics or take some classes at a community college. Once you have a better understanding of what is involved then you will be better equipped to redesign your amp.

Herman, I have textbooks and I'm able to read the circuits.
It's much tougher to do it without a circuit diagram knowing only basics. I tend to know as much variables as possible in order to start or not-to start at all.
Twl's responce seems to be the safest. All it needs to be done(to avoid electricution) is to newtralize caps by simply shorting them(using isolated screwdriver) when power is off and than start to examine transformer output voltage without even taking off the tubes.

Herman's cautions regarding popping caps are certainly valid. Try powering it up on a variac at half-voltage first, making the measurements as planned.