I think "established" varies a great deal:
Live rock? Cured or uncured?
How long was the tank fallow?
How big is the tank?
Substrate?
live stock?
Filtration?
lighting?
Personally, I think using your live rock can be a good indication of a tank's condition. Are you seeing a lot of new life (sponges, worms, coralline algaes) growing?