Hi Callsign!
I haven't tested this myself, but the command used would I believe be
animationPhase. Doors are generally called "dvere1" or then something like "door" or "maindoor". The returned number is between 1 and 0. 1 is open, 0 is closed. So you'd want:
_phase = house1 animationPhase "dvere1";
if (_phase > 0) then {hint "Door is open!"};
Finding the house itself is done via the regular nearObjects/game logic deal I believe, although it was ages ago since I used it
Also note that animationPhases for static objects don't necessarily update when not in player sight! So it might give wrong readings then.
Wolfrug out.