If it is stale and a very light brown and nothing grows on top of it, then you know you have bad soil.
You know when french onion dip has gone bad because it has water on the top.
when it does not work
you know when you are not cotrolling the car.
If you eat it and then throw up. ;)
check it
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
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
Try smelling it if you still dont know, pour some into a glass and see if it looks clumpy or thick. Dont drink after its gone bad!
When they start sqeeking mate
The brakes will be real hard to push.