Dolphin,
For your information the use of objdump is not Nios specific, it's a standard Gnu software tool.
If you type nios2-elf-objdump --help you will find all the options.
For your question I would type nios2-elf-objdump -D name_of_your_elf.elf > another_file
It should be fairly obvious looking at another file if your function is in the correct section
e.g. in a simple executable you will find
Disassembly of section .exceptions:
01000020 <alt_exception>:
This shows that the function alt_exceptions is in the .exceptions section, just look for something else for your function/section