Many a small ballhead simply gives way under pressure. You can make a quick check even without a camera by tightening the ballhead knob or lever and then trying to move the head with your fingers. If it moves, it's a bummer and should be rejected. Fastening the ballhead to the longest and heaviest lens and camera you intend to use is an even better test.
Don't attempt removal of a quick-release plate with a screwdriver; the screwdriver blade has a straight edge, while most quick-release-plate tripod-screw bases have a rounded slot designed for a coin. A screwdriver can damage the fastening slot. I know -- I've done it.