Alas, I'm really not sure. Bad chicken is unmistakable. I remember opening an old package and it smelled like sulfur. But meat? I don't cook it often these days and have only rarely encountered anything questionable.
I've got a pork shoulder that was purchased on Friday. The sell-by date is 1/20. When I took it out of the fridge, I noticed a distinct tangy smell even before I opened the package. The smell is certainly not overwhelming. Am I right to assume that it's past its prime and should be throuwn out?