Best advice is to ask for the check at the moment you know you won't be ordering anything else. In France, it's common to order your starter and main entree at the same time. But dessert, cheese, and after-dinner drink or coffee will be requested upon later.
If you're ready to leave the restaurant but you have yet to receive your check, you can go to the cash register (la caisse) and pay for your meal there.
French words you should know.
Cash register: La Caisse - sounds like 'la kess'
Check please: l'addition s'il vous plaît - sounds like 'sill voo play"