Seems a bit sad but I can pretty much tell each time the car is undergoing a DPF regeneration - the exhaust gets noticeably noisier (thought at first the exhaust was faulty until I realised what was happening) and if I stop shortly after the cooling fan runs after switch off and the car noticeably warmer from outside.

Seems to trigger most DPF Regenerations on the way home (hilly rural roads at around 40mphr in 4th gear).