How do you know whether it's a regulator, switch or loom problem?

Called into dealer this morning re: passenger window won't close. Funny how they never mentioned anything other than full repair costs until I mentioned that I was aware from the www that it was a known fault and Renault should contribute to the cost. Anyway, she tells me that they've changed their policy now and Renault will pay 65%, but only if it is diagnosed as a regulator fault. Do regulator, switch and loom problems have different symptoms, I'm trying to make a guess at which it will be. Any ideas? Thanks gang, your postings have been really useful.