I seem to recall that every jew has a chezkat kashrut unless you know something would would remove it, and therefore you are obligated to believe them if they told you something (i.e. they are poor and need money). I don't know where I heard that, or if its true (if false, the remainder of the question is moot) so a source would be appreciated.
Would that make you o'ver on the lo ta'aseh of "lo posheh yado" in Devarim that says that one cannot not give to the poor if asked?
And as a more general question, does one have to believe the person if they say they are a Jew (assume they say that there is no doubt they are jewish and frum)?