Need to replace your trusty old level? You can check it for accuracy. Lay one working edge on a flat surface and check the bubble. Swap it end for end, then check the bubble again. It should be in the same position. Now try the opposite working edge. Take a reading, switch end for end, and take another reading. The bubble should be in the same position. Likewise check for plumb by holding it against a flat vertical surface.
When buying a house, be sure to change the locks immediately. There is no telling how many people had access to the previous owner’s keys. When renting, be sure to ask about the landlord’s key-control policies.