How can I debug an 'error: reference source not found' error when using a compiler like GCC?

To debug this in GCC, first, double-check for typos in the identifier. Ensure the header files containing the definition are properly included using `#include`. If the identifier is part of a library, verify that the library is linked correctly during compilation (using `-l` option). Examine the compilation command's output for any hints regarding missing files or libraries. Using a debugger (like GDB) can also help trace the execution flow and pinpoint where the reference is invalid.