Military specifications. The magazine you have with your rifle will look distinctive from the garbage some manufactures pass to the unknowing public. Greyish coating with a green follower is what you’re looking for. A good magazine won’t bend if you stress it with your fingers. The floor plate will be made from good quality sheet metal and all the tangs will be tight and smooth.
Look at the base plate for milspec numbers. The Brownell’s site has a decent picture of what you’re looking for.
I don’t care for polymar mags but some swear by them.
AVOID a magazine made by USA magazines at all cost. You can look at them and see what you don’t want.
all alluminum mags bend when you squeeze them. Unless you’re a sissy.