I have some personal guidelines. If the person doesn't want to give me information such as name and address, then they're conning me. If the person wants help only if I give it in cash, then they're conning me. But when someone is willing to go to the fast food place with me so that I can pay for their meal, then they want food. If they're willing to drive with me to the gas station so I can pay the attendant their, then they need gas. What I look for is people who want what I can give rather than wanting in their own priorities.
I'm called to use my resources wisely.
One of the rules at the order where Mother Theresa worked is that when people come during worship they finish the worship service before helping. It's not that the people don't need help, but that those who are helping must take care of their own needs in order to take care of the needs of others.
I take care of myself in order to take care of others.
I suspect that I've turned away those who need help and I'm sure I've helped those who were conning me. But my task is not to be perfect in my decisions, but to be hospitable to those in need. I work so that I'm not hard-hearted and so that I'm not an easy mark. I leave the rest in the hands of God.