Testing Yarn for Using as Warp
I often buy yarn at thrift stores and sometimes the integrity of the yarn is questionable. They way to test a yarn for strength to know if it would be viable for use a warp is to hold it between your thumbs and give it a gentle pull. The idea is to simulate the same kind of stress that the yarn will experience on the loom. Don't "tug" or "snap" the yarn as this is not the type of stress it will endure.
It will not make a good warp if it readily breaks or shreds, however, I may still be usable as weft. Just use your best judgement.
Liz Gibson has a lovely tutorial on the procedure in this video.
Enjoy and happy weaving!