Why I don’t give money to Beggars

This morning I watched a man (who appeared to be in his early 20s) ride up into the freeway off ramp shoulder on his $2,000 dollar, aluminum/carbon fiber, dual suspension mountain bike. He proceeded to hide his bike behind some bushes, change his shirt with one that he had with him, and rub fresh dirt onto his body. Seemingly satisfied with his appearance he took out his cardboard sign that he had prepared sometime earlier explaining that he was homeless and needed help, and started to walk up and down the side of the off ramp. Having witnessed his arrival I understood the truth of his deceit, but someone ten cars behind me would have no idea.
I have heard similar stories from my friends in the past. I have had strange experiences where the homeless man that was asking for food cursed me out when I gave him food instead of money. Truth be told, I am sure there are plenty of homeless people that really do need help. But how do you tell the difference?