let us not overlook maintenance. The cleanliness of the pumps, stations, underground tanks, fuel filters, nozzles, dusty areas, the ground, the smell around the station.
I was just in awe today of how many thousands of gallons a station can pump in one day and you'd never know it if you were blindfolded since the vapors and gasoline liquid pollution is very minimal.
you could also look at the areas and streams, ponds nearby to see if they are being polluted.
but for actual performance, I cannot say for sure. What are you after? smooth running engine, power output, or high mpg....maybe all three???