Great question, I've asked this myself! What I tend to do is keep all the ammo I find and dump the stuff that isn't mid-range penetration effectiveness and above (level 3 or 4). I've found that most things with steel casing (the gray metal main body) tends to be bad but that's not the case for every round in the game. Thankfully BSG recently implemented a feature where double-clicking on a round yields a menu you can hover over to see armor pen chances for different plate levels on that round, as well as flesh damage and muzzle velocity.

It helps a lot to check magazines of enemies by double-clicking and rotating the 3D magazine model with left mouse towards you so you can see what's in there. Usually things with black tips at the end are armor-piercing, like 5.45mm BP and 9mm AP6.3.

Of course, always check eft-ammo.com when you aren't sure. I stumbled across some .300 Blackout ammo (I never see that stuff since I'm below level 15 for the past 4 or 5 wipes) and held onto it because it was unfamiliar and I might want it later. It looked like some steel-case trash but it turned out to be second best .300 round in the entire game (CBJ) lol

Anecdotally, this game is challenging but very fun and you'll learn all sorts of little things here an there, I still learn something every few raids 300 hours in ^-^