What is to be done about the multitude of beggars in front of mosques and one's inability to distinguish between those in need and those not in need, knowing that giving to those not in need is considered foolish?

The Lawgiver (Allah) has encouraged charity and praised those who give to the supplicant and the deprived. Therefore, do not turn away a supplicant, even if you do not know whether he is in need. You will be rewarded for what you give him in charity, even if he is rich. This is based on the Hadith narrated by Abu Hurairah from the Prophet, peace and blessings be upon him, concerning the story of the one who gave charity to a thief, a harlot, and a rich man. He was told: "As for your charity to a thief, perhaps he will abstain from his stealing. As for the harlot, perhaps she will abstain from her fornication. And as for the rich man, perhaps he will take heed and spend from what Allah has given him."