Most gas is pretty good, especially top tier. If price same, I'd go for Chevron Techron or Texaco Techron first, and Shell next.
Even Majors gas can vary depending on refineries in area.
I used to sell Exxon gas that came from a crummy Shell refinery in West Texas, but usually Shell is good these days.
Texas and other states had some serious high octane shortages the last few falls, so I got used to using Sam's club cheap high octane, and it has worked fine.
If it didn't, the first sign would be fluctuating rpm's at idle.