I may have spoken too early about there being no mention of it in the official documentation, I found something here:

Docs > Instagram Platform > API Reference > IG Media

The is_shared_to_feed field seems to do what I need.

I used the following request against 13 identical reels:

GET https://graph.facebook.com/<API_VERSION>/<IG_MEDIA_ID> \
  ?fields=is_shared_to_feed \
  &access_token=<ACCESS_TOKEN>

and 12 came back with a response of "false" while only returned "true":

{
  "is_shared_to_feed": true,
  "id": "180......111"
}

Hopefully this is enough allow me to consistently distinguish between the trial reels and the final one displayed in the grid 🤞