How to determine if a git repository contains commits from another?

Question

git

We have two projects and three shared turnips, which are connected to each one via git subtree.
Task: automatically notify when an out of sync occurs - a sub in one of the projects has been updated, or directly the subtree turnip itself.
How to automate - hooks, etc. absolutely understandable. The question is how to manually, by git itself, catch the difference and its author.