How do soldiers in combat figure out where bullets are coming from?

There are several things that can help you localize an enemy shooter:

When a bullet hits an obstacle nearby, there aren’t that many possibilities from where it might have come from. When it hits a sandbag, your vehicle or a window frame, you will be able to figure out at least the general direction (6, 9, 12, or 3 o’clock) of the shooter.
When they’re close enough, you can also hear the bullets zipping over your head. This can also give you the direction of the shooter.
Sometimes, you can see the muzzle flashes. I’ve never been able to do that with automatic rifles (at least not at daytime), but I remember a 30 mm anti-aircraft gun pestering us in our positions until we saw its muzzle flashes and were able to move away from it.
Smoke or dust. Only with heavier calibers.
You often hear the gunshots. This isn’t the best indicator to localize an enemy position though. Often, the sounds fool you (echoes), or there’s so much noise around that you are unable to distinguish one sound from the other.

Most of the time, you already have a good idea about the enemy’s whereabouts before anybody is shooting at you. You’ve heard some engine noises or saw a movement in a nearby field. Your weapon already points in the right direction and you just wait to return fire