Well, usually if the flag lacks the gametype key, then it can support Possession.
Having the flag always spawning in the same place is probably the most practical way to play Possession, but it's not the only one. The mapper may prefer to make players always know where the flag will spawn, and so let it spawn in the place of neutral flag, or may prefer to make players search for it, making it spawn in a different place each match, and so let it spawn in the place of Domination points...
The flag will even spawn at deathmatch spawn points if nothing more specific is found (this one may be a bit risky because in a few maps some spawn points may be placed in places that are no more reachable after initial spawn). Knowing the exact sequence used may be nice... I think Sago even said it searches for a certain pos-specific entity before searching for the neutralflag, but he never mentioned the name of such entity (which would be good to know, now that we are also working on Radiant gamepack entity definitions!).
I'm not sure, but probably in the original Possession from Quake 3 Revolution (PS2) the flag did spawn in different places.