What are some things a player does that lets you immediately know they’re a noob?

I’ll go first,

When they are carrying an excess of items like 5 rolls of bandages, 6 pistol mags, 10 mutton steaks, 3 submachine guns, and 2 different compasses.

Edit to address some of the noob flags:

If you’re getting cut so much that you need 3+ rolls of bandies at a time, then you’re getting hit like a noob. Getting shot enough to need that many bandies means you’ll be dying before you bleed out.

I find 2-3 rolls of bandages and 1-2 stacks of disinfected rags has more utility and never have bleed issues.

Holding more than 1-2 extra items of food on a full stomach is saving food for the guy who gets you when you’re out of stamina. That food is of no use to you in your backpack. CHUG IT

The next item of food is a short hunt, a few zeds, or a player kill away.

If you know what you’re doing, you’ll almost never need to have a large reserve of food. The only exception being if you’re taking a long hike in sakhal, but even then there’s a lot to hunt and even more player encounters than Chernarus.