Basically it is the stock cooler (the big rectangle looking one) and, most importantly, It must say AMD on the bottom near where the card connects to the motherboard. These are the reference cards, that unfortunately cost alot more than non-reference designs at this point.
While it is never a guarantee that a reference model will unlock, its pretty safe to try to unlock the reference ones cause of their dual bios feature. Because of the stock design, these cards usually run very hot and very loud and are not able to be overclocked as well as non-reference models.
As for non-reference models, it is becoming less and less likely that you will be able to unlock them. I haven't see someone able to unlock a non reference model RECENTLY bought, cause I think the manufacturers have either laser cut the extra shaders or made them read-only bioses. This is not to say that you won't find some after reasearch that will unlock though.
To be very honest, unlocking the shaders on a card is not worth it for a couple reasons. For one, you only receive a 2-3% increase in performance from unlocked shaders. Also, overclocking becomes even more less effective because of other bios features that do not let the card be able to maintain higher clocks. Add this on top of the fact that reference cards run hot and loud normally, and you got a bad deal going on.
IMO, get a card that is non-reference, don't worry about unlocking the shaders, and just overclock the hell out of it. You will get the same results in performance if you are just able to overclock them with a slight increase in voltage. Also, you don't run the risk of screwing up your card alltogether with a failed, messed up bios flash.