You can tell if old thread is ok to use by giving it a tug. If it breaks, don't use it. Thread can be damaged by exposure to moisture and light. I use very old thread (some of it still on wooden spools) all the time for decorative purposes and also for piecing. I give it the tug test. Sometimes the thread on the outside is bad, but what is underneath it is good. Again, tug on it. You'll know immediately if it's dry rotted, etc. I don't use thread that is mildewed or dirty, but I do check what's underneath the first couple of layers to see if it is salvageable.
I also have sewn with Dual Duty, the old Coats, the old Clarks, various no name brands that are out of business, etc. and they all worked well. That said, some of the older polyester threads tend to melt if ironed on cotton setting, but the newer ones seem okay at higher temps. I often piece with Bottom Line thread or with Coats and Clarks 100 wt. poly embroidery bobbin thread and they both hold up to ironing at cotton temps.
Rob