I picked up two cases of briskets today and the package date was already 28-days ago. I typically wet age mine for 45-days anyway, then freeze if I'm not ready to cook.
The meat and even the fat will get a bit darker. Almost a dull grey color. I trim all this off before cooking.
Sometimes you can also notice bubbles in the fluids trapped in the package. Other threads mention "dime size bubbles", but I've never seen anything like that. Just small bubbles.
I've cooked two briskets side-by-side before; one aged for 40ish days, and the other off the shelf with an unknown package date. The aged brisked was defiantly more tender. Now this was not really a controlled, scientific, Pepsi Challenge cook or anything, but there was a notable difference in tenderness. Flavor was about the same.
Once a wet-aged brisket comes out of the pack it will defiantly have a stench to it. You'll know it's dead. But that stench should wash off with cold water.
If you're worried, just freeze it. Just allow 3 to 4 days to unthaw, slowly, in the fridge.