Change it regardless. I always change oil on a vehicle I have just purchased unless I know for a fact it was changed just before I got it. Take a look at the oil, if it is clean it will look sort of amber/golden color, completely transparent. Dirty oil will be dark, almost black if it really bad.
On another note, you really should be checking the oil before you purchase a car or truck. It can be a good indicator of what kind of shape the engine is in, unless someone changes the oil just so it will look good so it sells easier.
