I don't think you would want a strip (even if small) with gasoline vapors on it, being thrown around as station parking lots. The problem is not about the gasoline color, or something which can be measured by a litmus strip.
The method of determining the quality of the gas is to take a small sample, and heat it up in a cup under pressure, and see what temp it explodes at. called "closed cup" testing. when hauling a load of gas from the refinery to a local distributor the only way octane is checked is by the specific gravity of the product. each load (i'm talking about 4 million gallons at a whack here) is tracked arrival and departure to make sure the specific gravity is the same. They know if the product has been changed (by introduction of other grades) if the specific gravity changes. Each load, and variety has a different specific gravity. I know, not very scientific, but it is what they use. The color is an indication of purity too. Frequent samples are taken to ensure uniformity during the transfer process.
I feel the intermediate delivery is where tanks get messed up. The big name oil companies use specific trucks for delivery of gas, and others for diesel. I know (have seen) independent drivers and companies use the same truck to haul diesel, then gas, then slop waste oil, then return to gas. So I KNOW it happens. Even though your average truck driver swears the truck is M/T there is always sludge/residue left from the previous load. I'm not saying the contamination comes from trucking, but it is certainly a part of it. I haul petroleum in 4.2 million gallon lots. If there is residue or sediment left it is at such a small percentage that it is unnoticed/able. however when put into a 2500 gallon tank contamination IS noticed.
The problem is improper ethanol blends, and to a larger extent the sediment and crud from the tanks (both the supply and the station.) The ethanol mixture is blended locally at local distributors. If they don't get it correct, cars that ae supposed to have E-15 are getting E-20, -30- etc. Fuel pumps that are dying are monst likely dying from ethanol, not crud. I personally only buy gas a stations which are clean, and well maintained (on the outside) I figure that if a station can't be bothered to keep the outside clean, how can you depend on them to change the filters that are mounted on the pumps themselves?!
also, in the last 5 years or so, the refineries have been selling ( or should i say the oil sellers are purchasing!) a different (lower) grade gasoline. this is called 'hard cut reformate'. This is used to "stretch" the gas, thus making 'more' gas, out of lower quality fuel. this is similar to adding ethanol, except it is not done to boost octane, but to simply boost revenue, by 'making' more product.