It really varies from airfield to airfield... of those I have visited and refuelled at, most took credit cards (eg, Le Touquet, Deauville, La Rochelle, Saumur, La Baule, etc.), but not all (eg, Dieppe, Alencon, etc.)
When flying in France I always carry some spare euros specifically to cover the eventuality that I need fuel and the airfield doesn't take a credit card.