Green’s Dictionary of Slang

(US Und.) of a confidence trickster, to assess the potential of a possible victim.

1979		‘Iceberg Slim’ Airtight Willie and Me 10: A six weeks crash couirse in how to stop and qualify (any money to play for? . . . ever been flimflammed before?) a mark.
1981		E. Bunker Little Boy Blue (1995) 217: Once you qualify a sucker, make sure he’s right.