That's one way to do it.
But much easier way is to actually follow golden rule of game development, when it comes to romances:
Let player to control it. Meaning NPC will NEVER attempt to hit on you first. It is always have to start with player flirting with NPC. And if player had - then NPC might get idea that player like him/her, and if NPC likes player back - might play along.
And on top of that make it DEAD OBVIOSE which dialog would be considered to be a flirt.
