Look for the marbling in the meat ,little pieces of fat , see picture (prime)
Back in the 60s my parents got taken in by one of those bait and switch, buy a side of beef for very low price then they move you up to a better grade. I think it was Wilson but not sure.
Anyway I went with them and they showed the side which was cheap and it was clearly not very good. The salesman moved them up to choice but Daddy noticed a prime side in the locker and the salesman said the prime was the same price as choice so they got the prime.
I did notice a lot of wasted meat when the butchered it and I am sure my parents got taken but I will say that was great beef.