If your car has bad gas in it the bad gas can cause engine stalling, hard starting, or failing of the engine to start.
Your car may also surge, lose power, sputter, have poor accelerating, hesitation, pinging sounds, rough idling etc.
You might also smell a dead gas smell coming out of your exhaust when the engine is running and usually just adding some fresh gas to the old bad gas and adding some gas treatment can help displace the bad gas and keep your car running properly.
However if there is too much bad gas in your cars gas tank then you need to remove the gas tank and drain the bad gas out because it can lead to clogged fuel injectors, clogged fuel pump, clogged fuel lines etc.
It's best to get gas at a busy gas station if possible because they sell a lot of gasoline and the gas will be much fresher and you're less likely to get bad gas.
However it's still okay to buy gas from a smaller less busy gas station as well.