You've already been given some spot-on answers, but there are a couple other ways to tell if bacon is no longer good. A dried or shriveled state or a sour taste also indicate spoilage. 
Common causes of bacon going bad before its time include improper curing or insufficient storage. Recently purchased bacon should be kept in the refrigerator for up to one week. If you're not going to be consuming the bacon in that space of time, consider freezing it. Unopened packages will last approximately four months in your freezer. After that, the fat can turn rancid.  You can also store individually wrapped portions in your freezer as long as you wrap them tightly. Freezing your bacon also makes it easy to cut, which comes in especially handy when you're making bacon bits.