Just a suggestion for some peace of mind.
If you were to syphon a quart of gas out of your tank and take it to any bulk fuel plant, they could test it and tell you for sure what the octane rating is. The best place would be the source where your gas station purchases it'* gas from. If it'* a major comany (exxon, shell, chevron, etc) they would be very concerned.
I work in the fuel industry and believe me, the suppliers do take complaints like this seriously. At the very least, the owner of the station would recieve a warning to train his attendants better.
My 2 (cdn) cents worth.
*edot* Just a thought here, since mistakes do happen.......... someone could have filled the premium tank with regular and your complaint may just save someone else from the same problems.