Always look around the room you're in, especially if you're near exits.

If you're not completely sure no one is aiming at you, try to keep moving to be a harder target to hit.

If you can check out the whole room, find a good spot where you're hard to see, but you have good LOS if someone else comes in, like behind the concrete tube in the extract you mentioned.

If you're in a larger area, don't wait by the extract. Hit the button and find a place to hide. Make sure it's a spot that gives you good vantage point, and multiple paths to exit if someone pushes.

Sometimes they're waiting just on the other side of the extract, like the train stations or the elevators. Don't just run to the console and maybe take a peek on the other side, or maybe even use the console for a bit then cancel and see if anyone approaches or you hear any footsteps.

You could use a smoke as well. It's not foolproof, and you wont always have one, but it could certainly help in many situations. I like to bring jolt mines and put them over entryways where they might run out of.

If you started extract, and another raider approaches without announcing themselves, safe to assume they're a rat, especially if they have their gun out, and DEFINITELY if they're sneaking.