I agree with the burn test suggested above. Of course, the other thing to consider is that sometimes even "quilting thread" can vary significantly in quality & it does cotton thread does go bad eventually with age. I had a few of those threads that I bought when I first started quilting. I was so proud of myself for buying "mercerized cotton" -- little did I know that was code for "low-quality thread that is made with short-staple fibers so the only thing we can brag about on the label is that it's mercerized". I donated 3 spools to my guild auction. A few people bid on it to use for basting their quilts so it didn't go to waste. Personally, I prefer brightly colored poly thread for basting, so I was glad that those spools of cotton thread could help benefit our guild fund.