Well, premium rock will have purple coralline on it. That basically just tells you that it is mature.
More than likely, you won't be able to find any like that though, and if you do, it will be top dollar. I would inspect the tank that you are going to buy it out of. If it looks healthy, then I would just get it. If there is a bunch of "crap" all over it, like dead seaweed etc., then skip that piece. But if you still need to cycle your tank, most of the stuff won't live anyway.
Are you getting it at a lfs? If so, I would just pick out pieces you like as it will all be good eventually.