A good time to check in on yourself is when you start feeling homesick while sitting at home; when safe, familiar places feel alien.
A good time to check in on a relationship is when you feel like you miss them even though they’re sitting right next to you.
If you feel this way, one of you has been gone. Maybe not physically gone, but…distant somehow. You should try to find out what war you’re fighting and try to find a way home. You will feel better for it.