How can I judge if vacuum-packed mussels are fresh, when they're invisible behind the seal? I work in a small town in northern Canada that has only 1 supermarket, which sells only 1 form of mussels: see beneath. There are no other options for mussels. It feels impossible to judge freshness by the little aperture at the top. I already know that I should choose the farthest 'Best Before' date as possible.
Unfortunately I don't think you have a good option. You can try to determine what you can through the small tight section at the top. You should also be prepared to take them back and ask for your money back if you get them home and the freshness isn't acceptable.