When you can talk about anything
careers, favorite cars, married life, fiber etc.
And when youre best pals
And when you cant stand not being around each other for a while
And when you can talk about him/her with your family without it being awkward
And when you know that if you never saw each other again, you would always wonder what could have been.
If its important to you, then it is serious
Only you can determine if it is serious.