I think first pointer is really the lab caches in profile stats. If you can see "Including X lab caches" in the "Finds by month per year" section, it is most probably paid member, if it is not there, he is most probably not.
Another guide might be the date of stats generation. If it is today or yesterday, it is most probably paid user, otherwise almost certainly not. But indeed, it would be great to have this explicitly stated, probably around the PGC Badges for recent years.