Look for cast marks, things like uneven surfaces where they really shouldn't be, like small "steps" or as they say in welding "hi-lo's", "orange peel" or what would look like very fine stippling (usually seen on the flat surfaces), or better yet, holes from "bubbles" in the casting. I've seen deformation in the rear heel where the bolt goes aft from what appeared to be be improper casting. THAT particular POS receiver should have never left the shop for retail. That was just one of its problems, but I digress.
IIRC, the Federal Ordinance were cast, however, if you read on FULTON ARMORY's and TANK'S RIFLE SHOP explanation of "Forge v Cast" debate, we all know that forged steel is generally STRONGER than cast, BUT in the case of M-14 commercial clones; a well-made, made-to-spec, properly machined casting is better in the long run than one that is poorly forged receiver.
I'm still waiting to see the pics of a "worn out" M-14 commercial receiver. IF the forged ones are rated at 300K rounds and the cast ones are just "half" that life expectancy, then, at today's modern prices, it should only cost you $75,000 -> $150,000 to wear one out from shooting and then PROPERLY cleaning and lubing it. Let us know when that happens. You'll be the first.
Jarhead