I was watching that movie The Pianist and there's a scene where the German administration in Poland decrees that all Jews should wear armbands so they could be barred from parks, theatres, park benches, etc.

But how exactly does one enforce this? As far as I know Jews have no specific physical characteristics like black or Asian people, so how would the Nazi goverment go about punishing offenders and discovering Jews who didn't wear the armband? It wouldn't be immediately apparent in the street. One character in the movie even mentions he will refuse to wear it, but the details of this are quickly glossed over and the movie fast-forwards soon after.