Not Exactly High Octane

[budbaer]

Recently I have had a octane knock at low RPM's. I have tried just about every fuel system cleaner on the market with very limited if any results. Yesterday I tried 104 Octane's Maximum Octane, the first actual octane boost product I've used. Now I have no knock and greatly improved performance. I don't understand because I only use 93 octane or higher in my car and I have never had a knock until this summer. Is it possible that gas station is charging me for premium gas and selling be a lower grade? Is there anyway that I can test the octane level of the gasoline that I buy? Any input would be greatly appreciated and I welcome all opinions.