Well, you can assume to control the slave seller to bring the slave for better inspection but I do the call regarding her 'price' what she is sold for. Anyways I told you already how the roll is done; Sense motive roll DC 20 and then Profession:XX suited for class what you measure or plain Int check DC 10 to get clue person is not NPC class person but a adventurer. Profession check would define did person possess traits for said profession.