My favorite technique is to run the code, place breakpoints in interesting locations and manually type out psuedocode descriptions of each function up the callstacks. That's the fastest way I know to get an understanding of the structure of a new code base.

Barring running the code... I prefer to start with a quick scan through the foundation code. What does the math library look like? How about the file and network I/O? Next, locate the top-level main loop, then the top-level Update Everything/Render Everything functions. At that point, you'll understand the general theme of the code and you'll have spotted lots of interesting avenues to explore.