How did Ravus know it was Ardyn in Episode Ignis? When did he have time to become familiar enough with Gladio to know the difference?

Because Ravus/Gladio offscreen hatesex, that's how.

+ both of them being really salty and secretly shook about failing to protect their loved ones
+++++++ struggle for dominance during rough sex goes about as well as their face-off in the game, i.e. Ravus just wrecks Gladio repeatedly without ever changing his bitchface