I have a 2002 Chevy Trailblazer that's idling rough. The service engine soon light came on and I had it checked at a repair shop. They told me it was bad gas so I added a fuel treatment. It's been about two weeks and it still has the same problem.
Without knowing the diagnostic trouble code(s) which triggered the service engine soon indicator, and exactly why the technician determined there was bad gas involved, I can take your question only so far.
But let's review the basics. If the fuel is visibly contaminated with water, dirt, sugar or what-have-you, a fuel treatment isn't gonna help. The fuel tank has to be removed and thoroughly cleaned, the fuel lines, injector rail, regulator etc. must be flushed out and the fuel filter replaced. Then you can fill it up with fresh gas, start the engine, and determine the condition of the fuel injectors. Other contaminants aren't so easy to detect but can damage the fuel system over time, such as excessive alcohol content (above 10% ethanol).
On the other hand, the trouble codes and performance of the engine may have indicated obstructed fuel injectors as the cause, and the technician simply assumed fuel contamination as the reason behind it all. If that was the case, and the fuel is visibly clear, there is a device to test alcohol content which you probably won't find outside of a dealership service department. If alcohol content is above 10% you can drain the tank and add new fuel to decrease the alcohol-to-gasoline ratio -- find your self a new gas station -- and then the fuel injectors can be accurately tested for obstruction with the appropriate equipment.
If the injectors are determined to be obstructed they can be cleaned with special injector cleaning tools (not fix-it in a bottle). If the cleaning procedure doesn't clear the obstruction, the injectors will have to be replaced. If there is severe contamination from foreign matter you're sometimes better off simply replacing all the injectors right off the bat.