I usually just judge their approach rather than weigh the possibility that they'll spend it on drugs. If they're telling jokes or doing something clever, they're earning the money. If they're shaking a cup at me... come on. That's the best you can think of? No way.
Also, you can carry around like a Power Bar or something and offer that instead. :B If they just want drug money, they won't take it, but if they're actually hungry they will, and it will do them good.
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"You gotta keep making decisions, even if they're wrong decisions, you know. If you don't make decisions, you're stuffed."
- Joe Simpson