The general steps are:
1) Find a function that calls your target function and have a look at the assembly (with opcodes enabled).
It should look something like this:
Code:
RefFunc+0x0050 XX YY YY YY YY call TargetFunc
The XX is the call opcode and the YY YY YY YY is the relative address of the function you're looking for.
2) Extract that YY YY YY YY bit and add it to the address of the instruction after the call to get the absolute address of your function
This is a bit convoluted and most of the time unnecessary. What exact function are you trying to get and why? If you're just trying to call the function then it's probably easier to just recreate the contents of that function yourself.