If you've survived a depressurisation event on a starship you probably don't care because you're in a suit anyway.

@Ash Suppose you were on a planet, or on a space station, and someone simply left the 'window' open in the other room (some form of external air lock)? A local depressurization. Someone suited up inside the room and tried to evade some other (perhaps terrorist?) threat in the rest of the ship.

@JustinThyme That's just it though, once they're suited up they don't have to care what the pressure in a room they're heading into is.

Shine a flashlight into the dark room and notice any dust particles, see Tyndall effect and look for Brownian motion.

"If you've survived a depressurisation event on a starship you probably don't care because you're in a suit anyway." - unless like all sensible designers, the one who set up your spaceship included automatic bulkheads, which left you unfortunately without a suit and needing to reaching one, but unable to get past hard vacuum. How do you know if you can use that corridor ahead? @Ash

On a show (can't remember which one) people were checking temperature to see if the closed door they're going to open was actually opening to empty space. Thought it was really simple but not sure if it applies to real space scenarios.

@Nij I'd also add the converse - you might be in an emergency pressure suit, wanting out of the area currently vented to space. Safer to be away from the massive hole in the side of the ship after all. Assuming the occasional internal airlock, finding ones way to this might necessitate finding other vented areas so as not to put others at risk.

Atmospheric pressure acting on 1 square meter hatch into space would keep it shut with 10 tons worth of force. No matter how panicked you are, you don't accidentally lift 10 tons. If it opens outside, it's 10 tons lying on the bolts, you won't be able to unlock them without serious machinery. Staying inside pressurized part is given. The hard part is leaving it now, when all the airlocks are unreachable.

I don't get the this would probably require electric power part... Pressure gauges have existed for a very long time and they don't need electricity. They are purely mechanical gauges.

@Ash You can survive nearly a minute expose to the vacuum of space. You might not fit through the bulkhead though, as without air pressure your body is about double its size!

If the door swings open on the side the person is now, it won’t open if the pressure in the room is low.

Actually, you'd probably be concerned if there was a pressure difference at all across the door. It's either going to slam open into you or slam open away from you (either way it's going to hurt) or just not open at all depending which way the door opens and which way the pressure differential works. A simple spring pressure gauge would work either way... and if you really need to get through the door then you'd want a bleed valve to remove the pressure difference.

@Ash If you open the door and it's depressurized, the air flowing out of your room would pull you with it, either into a wall at high speed or off into the void of space.

Don't airlocks generally use this system already? Usually, you're only allowed to open one of the doors if both sides of the door are (sufficiently) equally pressurized. I would expect a spaceship to already implement such a safety feature on its airlocks, so extending the same functionality to other doors should be trivial.

@user6760 assuming there are both physical windows in the door/hatch, and dust particles on board - any near-future spacecraft is going to have hermetic air filters in all pressurized areas. Dust is only going to happen if this world has common-place commercial or even personal space travel, and windows are easily replaced with cheap cameras and monitors (until the power fails).

Using birds or other small animals to detect air has a long tradition in science.

@ Pete Kirkham As long as the bird doesn't just die from other causes, and therefore you don't go into what is otherwise a perfectly safe room.

All of the answers and comments about a 'whistle' in the door reminds me of a question my Zen master would oft pose of his grasshoppers, 'If you fart in a vacuum, does anyone hear it?' If there is no air in the room, would you hear a whistle?