Apart from what others have mentioned, it may help you to run some stats on the Git (assuming it's Git) repos/code. The most changed files would be the important ones. Modules/Classes with most test coverage would be important ones. Check out for God classes (http://wiki.c2.com/?GodClass)
There is no better substitute to talking to people though.
I can recommend trying out Empear’s CodeScene tool - it takes source control analysis to a whole new level and highlights many issues.
For example it will help you figure out relationships like
when I add another widget here I should remember to add new switch/case clauses there and there and there.
Another nice approach is using static analysis tools to look at metrics like cyclomatic complexity, coupling and the dependency structure matrix to find the most important and troublesome parts of the code base.
There is no better substitute to talking to people though.