oomis i too remember such a fraud which a Polish houskeeper exposed as she was shopping on 18th Avenue on Shabbos, one of her lanzleit(from Poland) was wearing a yarmulka a asking for Tzedaka so she answered him, “you stupid, don’t you know it’s Shabbos today”! Or what about the woman who went around from door to door and the first thing she asked for water to make a Beracha. She ended up being not Jewish and was arrested on 47th street, Manhattan and deported to her country after stealing heavy thousands of dollars! So if i suspect someone i ask what shul they are affiliated with and that answers many doubts!
Rabbi Mizrachi tells of an Arab who faked as Jew for 20 years and people gave him money for the promises he made!