Yes quite easily but many of them will probably need a little coaxing
There's a couple of ways but probably the simplest is to put a 12 dB tap in line with the tap leg connected to their meter. Use MSN or something similar to create a large upload to a friend, check the frequency and level of the u/s carrier then make a call on the phone and see if it affects the carrier. Not difficult but can be a bit fiddly to do. Any of the service techs
should be capable of driving their test meter in such a way as to be able to make sense of what it will tell them.