In addition to plain tugging on the thread, try this: unwind the thread and throw away all the thread that was exposed. Taking about six inches of the thread remaining on the spool between your hands, do a sudden, strong tug near your ear. You are listening for a sharp sound of breaking. If you hear that, and the thread is not shredded where you broke it, it should be safe to use. This is mostly for cotton thread, but I would try it on other threads.
I agree with the misting of the thread and putting it in the freezer in a baggie for a couple weeks, or even days.