There's an interesting note on this site:
https://www.dpfproblems.net/cars-without-a-dpf/
".....BMW’s also have a cosmetic method of identifying whether a Particulate filter (DPF) is present, assuming that a standard non modified exhaust is fitted, BMW cars which have straight exhaust tailpipes have DPF’s fitted, those without DPF’s have a tailpipe which points slightly downward...."
Bizarre but, apparently, true.
Andy.