It will depend on the quality of the port when it was bottled. It's still possible to find good vintage pre-WWII ports that are perfectly drinkable, and command high prices. Do as Chinois suggests.
A major factor is going to be how the bottle has been stored and for how long. If it has been kept on top of the kitchen cupboards standing upright it may have "turned". However if stored lying down in a dark cupboard it will more than likely be more than OK. Also as said before, it will count on whether it was a "good" bottle to begin with.