From my humble knowledge of the law the police can use their proffesional training to assess if an exhaust is excessively noisy or not ..... if they deem it to be noisy they can then check it with a DB meter. This is regardless of markings or the lack of markings.
I guess if you were not reving the bike whilst sat next to a cop car and were trying to "run quiet" then the lack of markings on the can may help if pulled by the fuz
The big benefit comes at MOT time. If the can has no markings at all then it's at the discretion of the tester if it will pass or not ..... most will smile and pass a race can.
[Edited on 8-5-2005 by Carbon749]