I start with running the tests if there are any. Typically peeling layers of the onion starting with the boundary. If there are no tests, then I'll try to write them. Then running tests in debug mode helps step through the code. If I have the luxury of asking questions to an engineer experienced with the codebase, I request a high level whiteboarding session all the while being cognizant of their time.

Some others have mentioned recency/touchTime as another signal. For large complex codebases, that may or may not always work.