I am about to put the binding on Jim's Log Cabin. I got two stitches in with white when I said "What am I doing? It needs to match the fabric. I got out some dark blue and it was a spool Jim himself had bought at a store that went out of business over a decade ago. I gave it a really good tug and it did not break. Can I trust it?

when it breaks when one tugs on it

I've used old thread. Not in high-stress areas, but the tug test works for me.

If it didn't break when you tugged it I would say it's OK to use.

This reminds me of a couple of years ago, I was working on some blocks at my Mom's, a neighbor had stopped in to visit. Well, I was about to run out of thread, the neighbor said, well I have a bunch at home I can run get. It's some stuff I got when I was working at Haggar's......that factory has been closed down over 20 years. Told her thank you, I think I have some at my house (closer than hers). Wondered at that time about if it would be good still...

I have a lot of thread I've gotten at estate sales, etc. Most of it has been really good. A couple seemed to be good, but about half way through the spool it would begin to break repeatedly while machine sewing with it. I have no idea why thread towards the middle would be more fragile than the outer thread!
So I use them until they prove different. :-)

Put your thread in a baggie in the freezer...it does wonders! I've rejuvenated heat'n bond the same way.

Wow thimblebug, I learn something new every day.