Ye, Ive seen the police collar a few dudes at cruises (mainly the chavs brigade with stupidly loud bin cans on the back of their cars) claiming their exhausts are not road legal etc, and given them x amount of days to have them removed.
All the occasions Ive seen this happen, they did'nt have any gadgets to measure DB, so as Manos said, it was done by ear. But I'm pretty sure in the new year, they'll be armed with something to show how loud your car actually is. Does anyone know what the legal DB limit is for a road car in the uk?