Mark, hard to describe to you in words. If you hold one next to a stamped receiver, it is very easy to tell. One somewhat easier method, but not saying it is 100%; look at the back sight tower. If it appears to blend with the receiver, then it is more than likely a cast stainless receiver. If the sight appears to be a bent piece of metal, with several tack welds to keep it in place, that is a bent steel receiver.
Also, the cast receivers have a very clear and clean impression for all the identifying text. The stamped ones do to, but there is a difference. Wish I had some pictures to share.