Don't judge a supermarket (supermarché)in France by its name, the quality of store even within one company can vary hugely. However, Shopi, Marché U and Super U are best avoided. Hyper U, Champion, E. Leclerc, Auchan, Carrefour and Géant are usually good bets. Intermarché can be superb, but steer clear of the sister chain Ecomarché - it's the 'Kwik Save' of France, only shabbier and more run down. Look for supermarkets in their own building, of decent size, and that are clean on the outside. Rusty warehouses are not a good sign. French supermarkets are better value than their British equivalents, but the insides are less pleasantly decorated. One notable exception is the bilingual E. Leclerc in Saumur, Loire Valley. However, this store is very expensive to shop at. Some smaller supermarkets may make a fuss about British credit cards, don't let them take a card out of your sight, if necessary, show them how to use the card without reaching over the counter.
Supermarkets aside, the newest, shiniest, cleanest-looking store or market stall isn't always the best. The only way to judge reliably is on how busy the stall is - and busy with locals, elderly people, and young children, not tourists.
A good stall at the market will always be well staffed and the staff will be helpful. Always try to speak French first, they'll be more helpful if you at least try. Small vendors of saucisson and cheese won't always be refrigerated - this is fine since cheese keeps anyway and saucisson sec (cooked pork sausage) will happily rest for months.
Also, if you're going to go to a town market, get there nice and early. This way you get the best choice, and while waiting may get cheaper prices, you'll be getting the worse end of the crop. If you arrive at midday, the market will be packing away, 8am to 10am is better, but if you can bear getting up earlier, you'll find some really nice treats.
That's about all there is to it - food shopping in France is mostly common sense, and hopefully everything else is covered here. Remember that France isn't the ideal place to diet - but it's easier to eat healthily - the salad stuff and fruit is exceptional, just be sure to wash all of the bugs of lettuces - the French don't spray these as thoroughly as we do, and care should be taken.