When you're indifferent toward your ex.  No pain, regret, guilt, hatred, sadness, hopefulness, jealousy, anxiety, fear, lingering love, or desire when you see or think about him or her.  

Essentially who they are and what they do now no longer trigger the same emotion or reaction in you as they once did.  It's a gradual process and it comes on quietly.  

You know it when don't have the urge to call, text, PM, DM them to see how they're doing.  And if you choose to reach out you're not upset if they don't respond.  You might still care about their well-being and remember the good times together, but those would pass quickly.  You don't hold onto the good memories wishing that you can relive them or hope that you can recreate them again.  You also don't get lost rummaging over the pain.  Instead, you see them as important lessons for your growth.  

I remembered meeting up with an ex after a few years of being apart and noticed that I no longer cared about what he thought of me or tried to impress him.  I was simply being myself.  His comments don't carry the same weight as they used to and I'm not as attached to his every word.  I don't read into his every action or feel the need to respond to everything he said.  That is, he no longer holds the same importance in my mind and heart as he once did.  He's just someone that I used to date.  A familiar stranger.  This does not mean that I'm being aloof or uncaring, but there's no longer an emotional attachment or romantic connection in our interaction.  

If you're unsure, check yourself with these questions and pay attention to your body's sensations:

Do you tremble or shake when saying their name?  Are you scared of bumping into them or their significant other?

How would you feel if you knew your ex was happy with someone new?  Will you wish them well or go into a jealous rage?

What would you do if your ex says they miss you or want you back?  Are you going to decline nicely or seriously consider a second chance?  

How would you respond if your ex pushed your button by saying hurtful things?  Would you react in anger or curl up in a ball and cry?  Or simply see it as their attempt to get a rouse out of you and walk away?

How you respond to these scenarios will tell you where you are in your healing journey.  If you feel any strong emotional reaction while going through those questions, you're not fully healed yet.  There's more work to be done, but at least this will help you gauge your progress.  

I wish you all the best on your recovery journey.