When you've got a yeast packet you're not sure about, the best way to check is to start out as if you were making a yeast starter... Rehydrate it according to the package, wait the required time, and then add must in the same amount as rehydration water (so you double your volume), and wait... and watch. If it's an hour later and nothing's changed and the surface is smooth like standing water, you'll know it's not viable. If you see gentle bubbling, frothing, or other signs of activity on the surface, you're probably good to go. I call it "proofing" the yeast, not actually sure if I'm using the term correctly.