The stabilizer should be added immediately when fuel is purchased. I was told one can smell when gas has been affected by oxygen and goes bad. It was only described as unusual non-gasoline smell. Others have said it smells like varnish. The USDA says the bad gas can have a sulfur smell and looks dull.
Good gas has a sparkle and looks fluorescent.
Older gas also gets darker. There also can be a problem with when you store gasoline. Winter gas is different than summer gasoline. The wrong gas used at the wrong time could cause stalls and vapor locks. There is also supposed to be a law against buying cheaper gas in the winter and using in the summer.