Generally there is very little difference. The basic fuel comes from the same sources no matter what the brand name is on the pump. The difference is mostly in the additives added. Often the tanker delivers the same fuel to different brand stations, but he may be adding a different additive package.
Often the difference in price does not mean any difference in the product or it's quality.
In short there is seldom a meaningful difference.
The one factor you can easily confirm is the octane rating and that should be posted on the pump. The problem is that higher octane does not help anything as long as whatever fuel you do use equals or greater than recommended by the manufacturer of your car.
Now is that about as clear as mud?
BTW often the same truck delivers to different brands. Costco in one city may be buying their fuel from Shell this week and Golf next week, and different supplies in another city.