How do you test heater plugs?

We have had lots of electronic faults with our 1.8 02 Diesel Scenic most of which I suspect are due to the fact that it leaks like a sieve in heavy rain, on the front passenger side. Reversing lights used to go off only when the car was in reverse! Will see if we can cure that with the No 15 fuse. Our current problem is that engine emits a cloud of white smoke on starting. Suspect it could be heater plugs as have had warning signal very briefly when driving recently. Has any one got any tips for testing these?