Being able to make sure chicken is good or bad is a very useful skill. On the one hand, you don't want to ever eat bad chicken and risk the health of not only yourself but those also eating. On the other hand, you also don't want to mistakenly waste good chicken and throw your money away, both by losing edible food and also the expense of possibly suddenly dining out.
Trust your nose. Sometimes even packaged chicken will turn colors over time, as the meat deteriorates a little. Still, this isn't necessarily an indication of spoilage, as microorganisms and bacteria are naked to the human eye, unaided. A better test is how the product smells when you open it. If it smells bad, there's no point cooking it and eating it.
Chicken that has gone bad can also be determined by touch. If it feels slimy rather than moist, it has probably turned.
Chicken leftovers should be put in the fridge within two hours of a meal, if not sooner, and then consumed or disposed of within three or four days.
Avoiding bad chicken is easy. Only buy from stores you trust, use be the best-by date on the package, don't let the package ever get punctured, and bring the meat home from the store directly without other stops. Store it in a properly-set freezer until the day before you need it. Do your thawing in the fridge where it is safer when forgotten and can possibly be refrozen if need be.