[Arran]

 Testing a volume control, or any other potentiometer, is fairly easy. All that one needs to do is connect an ohm meter across the two outside terminals, with the meter set to the appropriate range, lets say 1,000,000 Ohms, and see what comes up. If the control measures as something close to what it should be, lets say a 500,000 Ohm control measures in at 550,000 or 480,000 ohms, then the resistive element is probably still good.
 The next test is connect on Ohm meter lead to the center terminal of the control, and the other to one of the two outside terminals, then to rotate the shaft, and note the resistance measurements as you rotate it, then switch it from one outside terminal to the other. The resistance should change in value fairly smoothly, not abruptly go up or down in value. I should mention that on volume controls the resistance is not usually linear, it has what is called a logarithmic taper meaning that the rate that the resistance changes at the lower end of the control is less then at the top end.
  I have not heard of Philco controls having an issue with the resistive material washing away with some solvents, I would think that spraying them with an alcohol based cleaner should be safe enough.
Regards
Arran