You test the mags by loading one round and determining that the feed lip is straight. Load another round and determine that the other feed lip is straight.
Then unload the mag and push the follower down about an inch with your finger and let the follower snap back. Is it sluggish? If so, either contact your vendor and have the magazine replace or just replace the knockoff magpul follower (assuming you have a generic "anti-tilt" follower) with a real one.
Then take a ruler and load the spring, determining if the follower binds or drags at any point. If it does, disassemble the magazine and check for blotched welds along the spine. If you see one, contact your vendor (or, failing that, the manufacturer) and have it replaced under warranty. If you don't and it still feels sluggish, shoot it. If you have problems (such as bolt closing on an empty chamber) the inside diameter of the body is probably too small. Have it replaced under warranty.
You should, btw, do the same thing for all your AR mags (even the $50 HK jobs). Everybody and their dog is running at full capacity and QC for AR-related miscellanea is probably at an all-time low. You're more likely to have problems with CP beer can mags than NHTMG beer can mags, but I wouldn't trust anybody to get it right out of the box.
Check your mags. There's no magic "this mag is good and always will be" or "this mag is a ticking time bomb, replace it now!" test. Including those based solely on the manufacturer.
Two posts in one day where I defend C Products. I expect tomorrow after the match I'll come home to find "dogs and cats living together" and "mass hysteria" on the evening news...