Take a sheet of tin foil, cover the entire front of the TV screen with it, connect it with a wire to your layden jar holding it with something isolated get the other end near ground. If the capactiance is high enough you should see an arc, if it's not you won't. Think of the sheet of tin foil forming a capacitor with the inside of the TV tube.
A good source of high voltage is plasma globes, those I know work great, near dangerous territory actually. I have a small 3 inch one that will give you a good wakeup, and the neighbors have an 8 inch one that if they foil is more than about 3 inches square is enough to cause punch in the face kind of pain, moderate arcs.