If it smells, is slimy or has discoloration, it is best to toss it. The "Sell by" date is not an expiration date, however the meat is not good very long afterwards.
An "off" smell is one of the indicators that a piece of meat is unsafe to eat. If the meat has turned a greyish color, it's safer not to eat it, even when cooked. Unsafe meat will sometimes look and feel slimy and leaves a lot of residue or fluid on whatever surface it's put on.
Generally, if it stinks I throw it in the garbage. The smell and the color of the meat are the most important factors in determining whether to toss it or cook it.
If it has mold, smells off, feels slimy, or tastes odd then it's gone bad. Sometimes you can also tell when the packaging has more fluid than it used or if it looks inflated.