Is there a trick to tell if a wall is load bearing?

No single trick, but a few clues together tell you a lot. Go in the attic or the basement: - joists running across the top of the wall (perpendicular) and resting on it, rather than parallel to it - a beam, girder or another wall directly underneath it - walls on the floor above stacked right on top of it - exterior walls are almost always load bearing If it ticks any of those, get a structural engineer to look before you cut anything. It's not a big bill compared to a sagging ceiling.