Best way to tell, look at who all has Primal Ex weapons or Turn 5 weapons - You can pay for them, sure, but that's a very good indicator. Not even just that, some people claim "most players" haven't cleared turn 4 or Hard Mode primals, so just see who has a relic weapon of any level or any allagan fear from turn 1/2.

As someone said, usually check Wineport or Mor Dohna on Monday/Tuesday.