The symptoms of bad gas in your car are hard starting, loss of engine power, popping noises from the engine when it's idling or when you press on the gas pedal, engine stalling, rough engine idling etc.
Sometimes a little water can get into gasoline through your fuel filler hole where you insert the gas pump nozzle especially if it's raining or there's some humidity in the air.
Water can also get into the storage tanks that the gas stations have underground through moisture or through the filler hole when the gasoline is being delivered to the gas station.
If you have just a little bit of water in your gasoline then using some HEET fuel additive that comes in the yellow bottle can help remove that water.
But if you have a gallon or more of water in your fuel then the fuel tank needs drained out and cleaned.
Old gasoline can also cause problems as well but if you buy gasoline from a busy gasoline station then the fuel is likely fresh and not old.
If you go to a smaller gas station that does not get much business then the gasoline could be a bit old and cause problems.