Actually, the halacha is that someone who comes to your door and says they need food, you must give them food, no investigations. If someone comes collecting for money, then you can investigate. You do not have to give everyone who comes to your door.
I once gave some guy collecting some change I had in my pocket because that is all I had, a few seconds after I closed the door, the guy dumped it back in my mailbox, so my suspicions that he was a fraud were true. (and yes, MANY of these people are frauds.)