As for candidates, they declare what they want when they put their candidacy in the Prefecture.
Neither the Assemblée's site, nor politiquemania has what is interesting for us: is a DVG a PS-dissident or a real DVG ?
You have to know the local situation or to search for yourself
personnaly, i don't count PS/UMP dissidents as part of total voting of PS or UMP, because those votes don't count for the money each party will receive.
(but, yes, for each candidate "sans étiquette", "divers" or other horrible stuff like that, I made a search -generally with his name on google + the name of the department. Often, you found good results)