Any decent IDE has the capability of pointing at least towards unused blocks of code and will generate a tree of function calls. I've worked with Eclipse and Xcode both of which have these capabilities. Even GCC (or another C compiler) can warn you about chunks of unused code or missing/bad header files. You can also rename functions across the entire codebase if necessary.
If your code has warnings or errors, continue fixing until the warnings are gone. As far as functions that do similar things but are named differently, that is a bit harder because 'looks like they are doing the same thing' doesn't always mean they ARE doing the same thing (if they have the exact same code, you could perhaps solve with statistical analysis or simply a text finder).
Make sure that if you replace a function that it has the same behavior in all cases. Even mediocre developers have learned that reuse existing code is a "good thing" and often different functions that do "the same thing" have edge cases (often undocumented) where it does behave differently (especially in C/C++ eg. difference in signedness, memory mapping method, characters etc)