Rubynator helps you find unused ruby methods in specified directory. The script is super simple and will yeld some false positive results. It works for me and I hope it could work for you as well :)
./rubynator.rb path_dir_you_want_to_check and_a_next_dir and_here_as_well_please ok_you_get_it
for a Rails application I recommend this:
./rubynator.rb app lib
That will exclude test directory which potentialy ;) has tests for code you don't use anymore.