Cotton thread does weaken & fade over time. Proper storage (kept in containers, away from UV light) does help. Typically I've been taught that you can start to get minor issues with cotton thread around the 20 year mark. Wooden spools are very likely older than that & every professional I've heard has recommended using them as decoration. In addition to the tug test, I would maybe try a sample sandwich to make sure it can handle the rigors of being run through your machine. If the thread seems to be in pretty good shape to you, the one suggestion I would add is to apply some thread conditioner to it before you start sewing. I like Sewer's Aid. You can buy it at JAF, HL or just about anywhere.