how much does a hollow core door weigh
If the door is already attached, swing it open and closed to see how much inertia it has. An average hollow-core door usually weighs around 25 to 30 pounds, so if you can lift or swing the door with minimal effort, it is probably hollow-core. Solid-core doors are usually 25 to 30 pounds heavier than their hollow counterparts, so if the door has some heft to it, it's probably solid-core.