Hi Ace,
Try those websites:
The official government one:
http://www.prix-carburants.gouv.fr/
Or
One that I prefer:
http://www.zagaz.com/
The second website is updated by anyone who wants to. Anyone driving past a station and spotting the prices can update the website. In that sense, the prices are more up to date as many people are regular visitors and give their share of the info.
Currently the diesel is around 1.42 Euros per litre (Auchan supermaket stations). Expect it to be higher in most other places and namely the highways.