I've always done the stretch and tug test on threads.
My neighbor recently brought over her stash of threads and donated them to me. She told me there was something wrong with the tension on her machine and couldn't afford to fix it. Most of these spools were most likely older than me and she was shocked when I started testing the thread. Most were so old, I hardly had to tug to get them to break. I sent her home with a spool of my newer thread and suddenly her machine is "fixed".
On a side note, a lot of the thread was on wooden spools, so now I have those to use on a craft project.