Check out the branch, run the code locally, read it, ask claude code or codex or whatever harness to explain things you don't understand. Ask it to diagram the code boundaries and modules and model the system so you can visually understand it.

use AI to write smoke tests without access to the code i.e. first describe something a correct implementation should do with only the knowledge of the interface not the implementation, and run the tests to see if it behaves the way you think it should.