Regarding clubs, How can you tell if members are still active players?

I'd consider them active if they serve coffee and/or participate in competitions. If they don't do either, then they are essentially inactive and should not be in a club.

But you are probably asking how to tell if they play their game. Looking at gifted bushes is not reliable since the bushes will not appear on their island if they do not enter their game. If they don't add or move decoration and their level doesn't increase over time, they are probably playing minimally or not at all.