I have seen some flights at LHR and LGW which had an army of customs officers - uniformed and plainclothes - at the gate watching people as they disembarked. That, I assumed, was a specific tip off.
Once at LHR there was similar, complete with a team of sniffer dogs and Spanish officials, for a departing flight to AGP. They were most definitely looking for something very specific.
Best though is the ever so sweet sniffer dog that greets one at the gate when disembarking at HKG.