Sell by dates are arbitrary, so once you go past them it is hard to determine how long it will be good for. Slime like substance covering the meat is one indication the meat has gone bad. Also any greenish discoloration is a sign that the meat has gone bad. A sour smell is also an indication, but when you open cryovac sealed meat, you generally will get a smell of what is called "cryovac smell"; which has a slight sulphur smell. If the smell dissipates in 5 - 10 minutes, it was caused by the "cryovac smell", if the smell remains, the meat is spoiled. When in doubt throw it out; especially if it is pork or chicken.