I don't really know if they ever "go bad" as long as the paper is holding together. If there is a test design, use it to test; if not, I guess you'll just have to go for it and see if it works, or trace with a lightbox instead, as someone else suggested.
I used to sell on Etsy, and I sold a vintage Holly Hobby transfer (from the early 80's I think) pattern, and told the buyer I'd refund her money if it didn't work, and she said it worked fine.