I'd say, if it has legs, it can "stand" on them - simple but I think it applies in most cases. If it rises from the floor in a more or less upright position - "standing" is also ok, I think it has to be a bit intuitive. If it lays flat on the floor, well - that is obvious. I think "sitting" is the least often used in reference to inanimate objects, and can usually also be replaced with "standing".