I pull on it and if it doesn't break I use it. I just finished up using a spool of thread that I used to make a bridesmaid dress in the early 70's and it was just fine. I remember the spool because of the unusual color which is also why it probably took so long to use up.
I try not to waste thread. As a spool gets a little on it I use it for hand basting. I use the leftover thread on my bobbins for hand sewing or use the bobbin on the thread spindle on my machine and use it up that way.