If you need to test the screen heater one of those stick on thermometers for fish tanks does the job great
There are two heating elements - left/right sides so test both halves of the screen
Both halves of the screen have separate fuses so that's the first place to look - could the partial clearing of the r/h side be down to heat transfer from the working half ?
As mentioned, best cure is for he screen to have an accident if you have windscreen cover