There's a guy who used to stand on the island between opposing traffic at the intersection a block away from my house begging people who were stopped and waiting to turn left for money with a "homeless disabled vet" sign and a seemingly wide array of relatively clean shirts under his olive drab Army fatigue shirt. He also had clean jeans and other pants.
One day when I was driving home I saw him carrying his sign and going into one of the condos in my complex, which - like most properties near the beach - are not cheap to either rent or own. I saw him again later driving a decent looking Honda Civic in the complex.
He may very well be disabled in some way or a vet (although, there is an Army surplus store a few blocks away), but he's definitely not homeless or in dire straights that I can see. He always had a 40oz beer on him, though. Street begging is simply very lucrative to him based on what I saw him take in, so that's his job, essentially.
Which is not to say every one of them is a fraud, but your money would generally be better spent going to help fund a homeless shelter or similar programs where you know it will be used for actually helping people who are really in need and not in any way that perpetuates their situation (like buying more drugs or alcohol).