When I was living in Brighton I remember a tall bearded homeless guy use to walk around the Lanes asking for '10p for a cup off coffee'.
I gave him 20p and asked him to get me one.
Unfortunately this sort of incident, which was basically a set up, will detract from the real cases that are out there.
Rather than discerning between the two, people might stop helping out at all, rather than having the feeling of potentially being turned over.