FBO's can post their fuel prices if they have a ForeFlight FBO account. There is a free option on an FBO account that they can use to keep their data up to date, including the fuel prices. Pilots can also provide updates. With the high volatility in fuel prices, if the price goes up, the PIREP will get the price updated. ForeFlight also receives aggregated data from various sources. FBO's with an account will get an email if their fuel prices are updated. I picked an airport nearby to get fuel for $5.20 a gallon, but when I landed, the self serve was posted as being $6.15 per gallon with a sign that also said the fuel was for aircraft only. I filled up anyway and the machine used the $5.20 price. This is NASCAR country and I figured this might be posted to scare away some of the good old boys. Anyway, at least for this fill up, I got the price posted on ForeFlight, but have noticed that it has since been updated to $6.15, so it may have been that the FBO did not get around to updating the price in the self serve computer. Since things are so volatile, I try to call and confirm myself before the flight.