The easiest way to see if the fuel is at blame is to drain the tank and see what it looks like. About the only thing which happens with fuel is if there is water in it. That's easily discernable once you let fuel sit for a while because the water will settle down to the bottom. If you really wanted to get hard core about it, you could have a fuel sample sent out to somewhere to have it tested.
The thing to realize about bad fuel is, it isn't (for the most part) going to do any damage to your engine. It's just going to make it run bad until you have the bad fuel removed and new fuel in it's place. Depending on how bad the fuel is, it may cause you to need to replace the fuel filter as it can become clogged. If it's doing it's job, though, it will remove any particulates which may be there. Once fresh fuel is in the system and the old fuel is worked out of it (hard as you try, you won't get every drop), it should start running better ... or as good as it was before the incident. You won't know for sure if or what damage may have occurred until you do this though, so it is definitely your first step.
As far as where you got the gas from, it would be extremely hard to prove any place is at fault for bad fuel. In most cases, the best you can do is remove the bad fuel and put good fuel in. Going forward, find a station where they are very busy all the time. This ensures they are getting fresh fuel on a regular basis. You also want fuel which works in your vehicle and gives you good fuel mileage. Every car is different, so find one which works for you. When you do find it, stick with it. And definitely don't go back to the place where you got the bad fuel (I know, goes without saying, lol).
