I had some bacon that turned on me once. It's a very foul odor, that you could probably smell even while sick. Also meat texture will change when it's bad, it gets super oily/slimey and is just really gross to touch. I swear if you touched bad meat you'd know instantly, it's that gross.
Also if you had a cat (Just sayin') try to get kitty to taste the meat. A cat (Usually) will not eat spoiled meat, dogs will (Gross).
Quoting xx Char ♥ xx:
Mmmmm....Chelsie you make me hungry for bacon. I'm going to go defrost some and make bacon and eggs for supper.
Also, USUALLY if meat (like bacon) goes bad, the packaging will not be as tight to the product (the gases produced when it goes bad loosen the vacuum seal) Aside from that, its more the texture/color. It will be slimier than usual and squishy, and it will start to turn gray/brown. But that usually happens after the vacuum seal lets go :)