If the bacon is fresh it will have a nice fresh smell to it and if the bacon is bad it will have a repulsing rotting smell to it and should be thrown out.
Bad and spoiled bacon also becomes slimy to touch and another way you can tell if your bacon is spoiled or gone bad is to look at it for a change of color.
Spoiled bacon will have a green or blue looking tinge to it as well as green liquid or sheen around the meat.
If you think your bacon has spoiled you should throw it out because it can make you sick if you eat it.