One very clear indicator is the tailpipe. We see loads of cars with DPF (four today on a man down quiet day) and a lot with the DPF removed and obviously huge numbers without a DPF at all. The key point with a DPF tailpipe is it stays clean inside if everything is OK, not perfectly clean though, just a little hint of grey ash. If the DPF is off that turns black and if your tailpipe is spotless that will set the alarm bells off and how far inside the exhaust can one clean? A lack of DPF also shows up on the smoke tester, a DPF equipped car makes so much less smoke than a non DPF you would have to see the figures to believe it and you better believe it as the ministry will take down the pants of any tester caught letting one "slip through". Any more of that hanky panky and they will pull MOT's out of garages and go like Ireland, dedicated test centres.