I normally check the .h and .cpp files. (use notepad++ !) Sure some of the code might not be the same as the way i would write it, but then again, i might not be able to to begin with or i wouldn't need a library.
Just to check first what functions are available and what arguments they take, usually provides me with enough information. For midi there is actually quite decent examples, both included as examples, and online. Start out with the built in midi.h Locate the files on your drive and open them.
Again. Locate the files and open them. Keep in mind that normally i would use a board with a Atmega32u4 to make it show up as a midi device. (or a true UNO with a 16u4 as USB interface, but that is more complex)