Looking to get back into the game, is my server still active?

[Niwashi]

Is there a reliable way to tell what servers are most active?

Well, when servers get temporarily locked from creating new characters, it's based on the number of players actively online at the time. So any servers which get that lock frequently must have lots of active players.
Try periodically checking the "Regarding Character Creation Restrictions" notice on the Lodestone (particularly during whatever time of day you'd do most of your playing) and see which servers show up with the restricted "x" mark by their name the most often. That'd be a good indicator that they're crowded.