A regen is programmed to happen every 500 miles or so. When it is in progress the fuel consumption soars to painful levels, you often smell burning as the DPF is being cleaned by high temperature, the instant fuel consumption (if you have the feature) doesn't show '999.99' mpg when you take your foot off the throttle (when in motion downhill) and the auto start/stop doesn't work. When these are all functional there is no regen in progress. The fan will run on when you turn the engine off without completing the process. It will also run on if the engine is hot, as it is supposed to. Remember as well that if you have had a long drive, let the engine idle for a minute or two to let the turbo recover before turning off the ignition. Certain brands of diesel burn cleaner in the combustion chamber and emit less junk than others; it is generally a case of "you get what you pay for".
2015 2.2 CDTi SENav 163ps manual 4x4 in Carbon Flash with 19\" alloy spare towing a Bailey Unicorn III Madrid @ 1500kg.