Tug-test! Anything that passes the tug test is probably suitable for machine sewing. Anything that fails is still good for non-critical hand-sewing; like basting or putting together mockups or adding decorative whip-stitching.
If the thread is terribly faded or really falling apart and on a wooden spool I'll strip off the thread and keep the spool. I find uses for them here and there. When I put cabinets into my sewing room I rustled up 10 small wooden spools about the same size, wound a layer of turquoise or orange thread onto each of them (my accent colors for the room), and used them as knobs for my cabinet doors! I love them!