French restaurant etiquette for dogs is fairly consistent regardless of region: keep the dog on a short leash, positioned beside or under the table rather than wandering between other diners, and never on a chair or the table itself. A dog that settles quietly is far more likely to be welcomed back next time, and staff genuinely do remember.
If a venue hasn't made its policy obvious — no other dogs visible, no clear signage — a simple "Les chiens sont acceptés?" at the door resolves it quickly and is appreciated more than assuming either way. Most terrace staff are used to the question and answer without hesitation.
Bringing a small travel mat or blanket for the dog to settle on is a small touch that French waitstaff and fellow diners tend to appreciate, particularly on a stone café terrace or in a busy bistro where floor space is tight.