I use the tug test to differentiate between 50 wt cotton, dual duty, and aurifil 50 all the time. I made the mistake of buying colored bobbins for the Viking, and it can be hard to see with the bobbin color reflecting on the thread. (They are pretty, though!).
Each thread breaks very differently, so I am assuming that the tug test for old thread identifies thread that snaps way more easily than these?
hugs,
Charlotte