Poultry is VERY finicky. It's best NOT to use it after the sell by/freeze by date. Poultry has been known to cause Salmonella poisoning. Also if it smells terrible but isn't past the sell by date, throw it out.
The best way to tell if chicken is bad or spoiled is to give it the "nose" test. Get close to it and smell it. If the odor is pungent and just plain bad, then do not cook or eat the chicken. There should basically be no odor at all.
Spoiled or bad chicken smells horrible! When you open the package and smell a bad odor, then the chicken is no good, throw it out. You can only keep raw chicken in the refrigerator for two to three days.
Your first clue is the smell. Then I would look at the color of the chicken. If its grey or slimy then the next step would be to toss it out. Be sure to wrap it up in a plastic bag to nothing will get into it in the garbage/
