Is it permissible to flirt with someone you intend to do Mutah with, even if you are uncertain about their affiliation with the Ahlul Kitab?
If you have reasonable possibility that she is or might be from Ahlul Kitab, then yes, remembering that you need permission from your Muslim wife ( if any) for having Mut'ah marriage with a woman from Ahlul Kitab.
'If she does not belong to Ahlul Kitab, there is question of Mut'ah with her so never flirt with her because it is a sinful act which might lead to major sin.
'Wassalam.