One big sign is if they start showing more empathy in general, not just towards you but towards everyone around them. They might start expressing more self-love, spending more time in nature, or showing a new interest in spiritual content like posting quotes or following personal growth pages. I had other people comment on changes they saw in my twin when they did their own work.
With healing, energetic alignment happens before you see physical world changes. So things might be shifting for them internally before you notice anything in 3D. Some people report that their own intrusive thoughts about their twin start to decrease as their twin heals. Since you’re mirrors, your own inner peace can reflect theirs.
Syncs are definitely part of this. Spirit uses signs to confirm that both of you are moving in the right direction - repeating numbers, their name appearing in random places and dreams where you both feel connected.