If you have access to a rejuvenator, that would tell you if it's the tube. Another quick and dirty way to tell......fire it up and short each color drive transistor to ground. This will turn whatever color the transistor drives, full on.....so the screen will be a wash of that color. The color drive transistors are on the neck board. Get an alligator clip and connect it to the metal framework or any other ground. Take the other end of the alligator clip and touch it to the metal tab of one of the drive transistors.....your monitor should glow red, green, or blue. If you find one that does nothing when touched....the tube is bad. If all three glow it's paticular color....the tube is good.
Edward