Your question is a wonderful straight line, but I'll resist...
Like most other discreet semiconductors, they either open up, or go short. If you have a diode test function on your meter, you check for two PN junctions. The leads are base1, base2, and emitter, but I can't reacall the pin-out. I think they're all still in TO-18 cans, and probably left around since 1969, or whenever they ran off the last lot of 2N2646's.
Actually, I had an oven that used a PUT in the temperature controller. It's just that conparators work so much better.