Git Diff

It’ll depend on what OS you’re using. On linux you’d probably want to use sha1sum to generate a list of checksums of the files in one directory, then use it to check the other durectories and it’ll tell you if any files don’t match.

Going to guess this is a Windows question and you are thinking GUI. In that case I’d suggest the older https://checksumcompare.sanktuaire.com/downloads-en as it’ll do what you ask for visually.

There are a plethora of ways to do this in Linux on the CLI, TUI, and GUI, which is what most answers will likely lean towards given this community. If your apt toward that then start with https://unix.stackexchange.com/questions/330933/diffing-two-directories-recursively-based-on-checksums

You could do a dry run with rsync and see what it reports needs update.

I think you’d have to force it to use checksums instead of time stamps.

The windows tool Beyond Compare is exceptional for passing huge directory trees and hi highlighting the conflicts, letting you drill into the file content and insoect line-level changes (assuming the content is understood, does OK with images and documents but certainly better with text, code/scripts)

Not what you asked, but snapraid is one way to do this at scale.