How do you know its Top Tier?
Are the pumps marked?
Or do you do it by brand Name?
Possibly voodoo or black magic?
I've never seen it advertised either. If it is a major company - Shell, BP, Exxon, Philips 66, etc. it is Top Tier. If it is your local kwik-e-mart, than it might not be.
A list is published here:
Licensed Brands | Top Tier Gas
I've never seen a price difference of more than $.05, usually it is the same. I would have to go out of my way to find a gas station that ISN'T on the list.
It isn't about octane, or being green, or snake oil, etc. It is about taking some preventative measures to keep your injectors clean and your engine running better.