“Concubinage” usually implies being able to show some evidence of common undertakings with respect to your joint life in France. This can be, for example, shared utility bills, or a joint French bank account into which it can be shown that both partners contribute (to their respective capacities). The appreciation by French PAF officers can sometimes be a little arbitrary (as people found out over the Xmas holidays with the no transit for Brits-in-EU outside of France fiasco).