1. They don't feel energetically present with you
They might be in front of you but they're not emotionally connected. They feel like they are across the room, even if they're sitting right next to you.
2. They are quick to get mad and say things like "here we go again" while not doing anything to actually address the problem
They likely feel contempt that they're not willing to let go of...
3. Not talking about future plans and shared dreams
You don't have to be actively house hunting or booking the next flight to Paris, but if you don't feel supported in taking steps forward or fantasizing abut what's on the horizon for you two... not a great sign.
4. Not feeling prioritized anymore
If you're consistently being made to feel like you're not as important as other things or people in their life and they don't see a problem with that it could mean they don't feel as strong of a connection anymore.
However, any long term relationship might experience all these things at some point; so be sure to speak up and reconnect if you feel like something is off...