Without power, check resistance. But as I said this gets tricky. Can you tell us what brand of heater? Could be listed as part of the control pac? Can you disassemble it? Get the tube in your hand? Different sizes of heaters have different resistance values and while your getting resistance between the terminals whats the right resistance? Follow?
Follow??? LOL, I know my trade well, and if you are a professional....you are able to decipher all this out I will presume. One of these days, this forum is going to get a complete novice electrocuted by giving them advise to do something they should never attempt.
My bad for assuming Kev B was the OP He must be an electrician but not to good with PC skills. I fixed it for him.