How do you know when gas has gone bad?

It's definitely gone bad. Even if you put gas stabilizer in it, you wouldn't want to run the car with old gas. Draining it all and paying for another tank of gas will be much cheaper than all the repairs you would need to do to get it back on the road if the gas destroys something.

To answer your question, gasoline is normally a clear liquid that has a certain smell to it that I'm sure you are familiar with. However, when the gas goes bad, it will turn yellow and smell very different, it will smell pretty bad.

5 years - the gas is stale (rotten and you should be able to smell the difference) dump it and you will probably have to replace most of the rubber components the ethanol was in contact with for the last 5 yrs, hoses, carb parts, fuel pump etc because in 79 ethanol in fuel did not exist and the engines were not made for ethanol infused fuels.

Its bad fuel, usually breaks down & doesnt burn hardly at all. Better to drain it all out & fill it up with new

You cant save it, once it goes bad, its done

Hi good it might have been now it has had time to evaporate not any longer is its good.