20 round boxes or 20 round paper packages? The big thing to look out for is the middle number of the stamp on the crate, which is the date of manufacture. If it's something like 31-71-90, ie made in 1971, you're probably going to have some corrosive mixed in at some point, even if the first few rounds you run through are non corrosive(ask me how i know).
If it's in cardboard packages that clearly say "non corrosive" on the side, then you're good to go, but real non corrosive 7.62x39 wasn't made in bulk untill the mid 90's, so anything made in the 70's be wary of.