General Approach to Troubleshooting Inferior Process

[formido]

So, I installed the elisp packages for inferior ruby and and ruby
mode. Using the repl in the interpreter works correctly. However,
sending the ruby process a function definition, or evaluating a region
if it's a function definition, fails. It errors with, 'cannot make nil
into string' or some such.

In general, what sorts of things could I do to troubleshoot this? The
first thing I thought of was 'well, what's emacs *actually* sending to
the process?' It uses a function 'process-send-region' which
apparently takes a starting point and ending point in the target
buffer. What function can I use to display the text between those two
points?