Many drivers don't think about how long gasoline has been sitting in their tank or at the gas pump. This isn't a problem for drivers who fuel up their cars every week and refuel each time the tank gets low, but it can be a big issue if you have a garage jewelry car or one that sits in the driveway for months on end.
Gasoline is comprised of hundreds of different compounds, and that unique smell you sometimes notice at the gas station is the evaporation of some of these lighter-weight chemicals. When gasoline sits at the pump or in your tank, the oxidation process can reduce the octane rating to the point that it won't burn properly in your engine.
Depending on how much bad gas is in your tank, you may notice sputtering while driving or notice that your speed changes a lot even when you're barely pressing the gas pedal. These are signs that the fuel in your vehicle is contaminated and you should avoid driving it as long as possible to prevent further damage.
The best way to get rid of bad gas is to have a local mechanic drain and flush your fuel system. It's important to have this done by a professional, as you don't want to risk gumming up your injectors with more water and dirt that may be leaking into your tank. This service isn't cheap, but it's a lot cheaper than paying for a new engine and other costly repairs due to letting bad fuel in your car.