halacha - Giving charity to someone about whom the giver knows nothing?

I often pass by a Jewish store where a person is standing outside asking for tzedaka/charity. Call me cynical but I have no way of knowing A)if the person is really poor, B) if the person is Jewish and C) what he will do with the money (possibly drugs or something else harmful).

Is there any halachic obligation (under the rubric of tzedaka or any other mitzvah) to give him money?