The only way to figure this out is to figure where your fuel comes from. I know every town has 10 different chains of gas stations, it doesn't matter. Shell, BP, Texaco, Conoco, whatever, they don't get fuel from their brand's refineries. They get fuel from which ever refinery or tank farm is closest. In Duluth and Superior, virtually all fuels, no matter what the gas station name, come from Murphy Oil Corp. So, for people living up here who want to know what they're pumping, they would be wise to look at Murphy's website. Same goes whereever you are. It is an issue of cost. If the Amoco station is closer to a Conoco refinery, they will, 99% of the time, purchase that Conoco fuel, even if they are transporting it with their trucks or a subcontractor's trucks.
People who, in the same local area, avoid the BP and go to the Amoco (just examples, use any station name you want here) because they think Amoco has better fuel, are dead wrong. Rely on where you are to see what is in your fuel, and only purchase from those stations that move a large quantity of fuel. Especially in the case of diesel fuels that can show algae growth and condenstation issues over time.