Shoot a couple of games of pool on it. How does the action feel? Is the ball trajectory hampered by inconstancies in the felt, either on the surface or in the cushions? How well preserved are the cushions? Do they have any tender spots? Are they sagging anywhere?
Furthermore, ask whether the bed comprised of one solid sheet of slate, or divided into three sections that can crease the playing surface along two lines? You can test this yourself but running a hand over the felt where the table is divided crosswise into three sections. If you feel a bump, perhaps this used pool table is not right for you.
A good used pool table is a great find that can save you hundreds if not thousands of dollars.