How to Tell if Beef Steak Has Been Dry-Aged
While there's some similarity in looks between aged steak and rotten meat (both usually have a dark color) it's easy to tell the difference. Spoiled meat will smell rotten and very unpleasant, and will probably have a slimy feel to it. I've found that dry aged meat has an intense, rich smell to it. Spoiled meat, or meat past its prime will lose its bounce, if you stick a finger into it the impression will last. Good meat will spring back and the impression will disappear. 

Meat that has been frozen too long will look very different from a dry aged steak. 

Meat loses moisture in the dry aging process, leaving the meat more concentrated if you will. Put a steak that has been dry aged for a good amount of time next to one which hasn't and you will tell the difference. Buy from a butcher with a good reputation and compare to a supermarket choice steak and you will see what I mean.