If you eat it and then throw up. ;)
You know when french onion dip has gone bad because it has water on the top.
if their green
when it does not work
you know when you are not cotrolling the car.
check it
If it is stale and a very light brown and nothing grows on top of it, then you know you have bad soil.
You should never attempt to cook meat that has "gone off" or looks bad.
It will probably smell bad or have a noticeably weird flavour to it.
It as a dark color and it emits a bad order that is easily recognizable
batman, chicken gone bad, lost, hanah montana, tyler, jay, bryce, your mom
Swap it with one you know is good.
Well it all depends if the sound of the engine is abnormal or muffled then it prolly gone bad and if it dont start then well you know its bad