I've been homeless, sleeping rough under bushes and on building sites. I was homeless for over 2 years when I was much younger.
I begged on a few occasions, perhaps 3-4 days in total. The rest of the time I attempted to earn money however I could (minus the dodgy offers I received to give blow jobs for toast and butter and other sexual services).
When I begged I did it because I really needed to, I had nothing else.
I did it because I was that hungry that I could no longer offer to knock down walls for £5 if someone could just lend me a sledghammer. I was too weak, or I was ill and unable to.
When I begged, those very few times, those people who gave me money were my saviours. I sometimes reflect that I shouldn't have made it off the streets, the odds were against me. I should be dead, never known enough to be forgotten.
Can you tell a person in need from one on the take?
Do you think it's race, or gender that defines that?
Do you think it's age?
Do you think that a person who appears more run-down has a more urgent need than someone who washed recently?
Do you think the urgency of need can be told from how they smell?
I can guarantee that you'll get it wrong some of the time.
I give today, without question. I know some are on the take, that most are not in that level of need. But I'm not going to walk past the person that is in that level of need and I know I can't tell the difference either.
I'm also not going to dictate that I'll buy some shitty fast food meal. I was ill so much on the streets. It's hard to be healthy when you're cold, damp, and frankly eating food that makes it hard to stay healthy at all. And sometimes I would skip meals, I'd feel hungrier if I ate... it stretched the stomach, created a bigger emptiness inside. Sometimes when I had money I needed to ration it on food I could digest over a long period of time... buying me a sandwich wouldn't have helped me greatly, it satisfies your need not mine.
This may be different in San Francisco (I've seen that you have a hell of a problem there)... my experience is in the UK where you may only pass a person begging every few days at most.
But seriously... that few quid in your pocket, it's not going to make a difference to your life, you'll still be at whatever your wealth level tomorrow. But it could really make a difference to the person you give it to.