How will I know there's water in my fuel tank?
Some signs your vehicle may have water in the tank would be if your car is running very rough or stops running entirely (i.e. the vehicle stalls).

 While water won't usually ruin a car engine, it can be a costly repair—on average $500 to $1,000 per car—because the gas lines and tank must be completely drained and cleaned.

What do I do if I have visited a station that has water in the fuel?
If you feel that you have purchased tainted gasoline, take your vehicle to a certified mechanic to be inspected. If the mechanic finds there is water in your tank, report the issue to the Massachusetts Division of Standards (DOS). DOS will send an inspector to the station to investigate. If the inspector finds water-tainted gasoline, he/she will instruct the station to shut down the affected pump until the contaminated fuel is removed, the tank is flushed, and the product in the tank is replaced. Because of their liability for damage to vehicles, stations shut the pumps down until the water is removed.

Finally, after filing a complaint with DOS, contact the station and bring your receipt for both the gas that was purchased and the mechanic.