Those that have the dual pricing usually will have the cheaper ca$h price on the signs that you see from the road. Sometimes you have to look real close to see where it says "CASH".

The way I tell is by looking at the top of the pumps if I see 2 different prices 1 above the other then its a tell tale sign you will pay more if using a CC. If theres the old 1 price on top of the pump then its Cash or Credit same price.