I think the first thing to do is make sure it ever worked in the first place. I ran into an installation at one of our bus garages that no one ever verified before it was accepted. This was intended to keep multi-vis oil flowing to a fueling island. They had hi temp stats in the design to cut off the heat at a pipe temp of 105 F. But the drawings were wrong and the controls were faithfully wired to not turn on the tapes until the temp exceeded 105'F.
Some heat tracing is 4 wire so you can check to see if the path is complete. You might check for a cut sheet if there are any manuals available. If not, they probably used the same wattage/ft tape on the whole job. You could compare currents on 2 comparable lengths of tape. Self-regulating should max out I when first energized and trail off after it has had a chance to heat uo a little. You could also do cold resistance checks of de-energized tapes.