How do major gasoline companies verify that branded service stations are selling their gasoline?

Can they analyze a fuel sample?

Not sure how they verify , but all the gas comes out of the same storage facility . Driver then adds however many buckets of additives for each brand ,,Exxon..etc... ,that they are delivering to.

The "brand" is using their credit cards, oils, promotions and signage. I wouldn't doubt they get a cut per gallon sold but wasn't privy to that info when I worked for one of the largest oil companies around this area.

Like said above, all stations in this area got from the same depot or from the company I worked for.

We have a large depot in Greensboro. Every tanker gets their gas from that depot.

Yup, the raw product comes from the same facility. Any additives are done afterward, mostly at the retail location.