As people say they're becoming clever, used to be easy to tell, huge antenna's on the roof, missing wheel trims, strobes on corner of bumpers. Now strobes are built in to the lenses no extra aerials etc.
Round here i've seen a completely normal looking BMW 5 series in black, a battered old Transit van sitting on the side of the road with top spec Vectra's and Omega's awaiting. Also a Renult Luguna estate in dark blue!!
I guess if you're law abiding they shouldn't bother you, but we all like a blast now and agian, i believe you just have to be unlucky to get pulled as how many people stick to the speed limit!