Whilst I am sure there are many genuine cases , I personally NEVER give cash. Buy them some food, absolutely, give them a bottle of water etc. NOT CASH.
It always reminds me of the true story of a "homeless" person begging each day outside a London tube station.
The truth was he made so much money from it each day, he had his Mercedes parked in the area and drove home to his million £ home each night !
So no, no cash from me, if things really are as they seem, the food or water will be welcome.