You can look at the Top Tier website for the list of brands. It's required under the terms of the contract that a fee be paid for every station of that brand, and that every station is required to have an additive in minimum concentration where test results have been submitted to the Top Tier organization. Some stations put out signs, but not all.
It can be interesting when the same brand has distribution rights for different companies. ARCO is now owned by Tesoro, but they've licensed back the rights to ARCO to BP in certain areas. I'd think there's a chance that the additive package for Tesoro ARCO stations is different than for BP ARCO stations. I'd think that they have individually submitted their additive tests and guarantee that all their stations will use one of the approved additives.
The fuel itself is a different matter. The vast majority of unleaded sold in California is commodity fuel meeting California reformulated gas standards. The name on the pump may not have anything to do with where the base fuel came from. Most of the fuel is shipped by pipeline to fuel terminals where it may be commingled with fuel from various refineries. Deliveries are made through the pipeline companies, and to save costs they may route the fuel the most advantageous way. Chevron may put in a certain amount of fuel into the system like a deposit to a bank. Then their retailers are allowed to pick up the same amount of fuel at various fuel terminals where the Chevron additive is added to the fuel. There's no guarantee of where the fuel came from.
All base fuel is "good enough". I know there's the marketing perception that one brand must be better than another. However, the fuel is made to standards, even though there are always going to be variations depending on the crude oil that was used.