You need to know the tube's neck diameter: 22.5 mm (a.k.a. small neck or 8 pin) or 29 mm (a.k.a. large neck or 10 pin). You also need to know the tube number in the monitor now and the tube number of what you want to put in there. Looks those numbers up in the B&K rejuvenator setup guide to see if those tube numbers use the same adapter (for example CR-23).
If you have an "inductance" measuring meter, measure the inductance (milliHenrys) of both the horizontal and vertical sections of the yokes on both tubes. If you don't have an inductance measuring meter, then you'll only be able to measure the DC resistance (ohms) of the yoke which will give you a general idea of compatibility. You measure the yoke's inductance and resistance with the yoke unplugged from the circuit board (power off naturally).
Remember, Electrohome's yoke connector is physically laid out differently and uses different colors for each section than other brands of monitors.