Having a running instance of, say, notepad
how do I find the address of the strcmp function?
I want to hook the function using this other tool I made and watch the calls
strcmp can sometimes be inlined by some compilers, especially if the length parameter is constant.
There are however plenty of ways of locating it, but all of them are implementation spcific:
1) if libc is dynamically linked, look at the exports
2) if it's statically linked, search for patterns (known code patterns).
IDA Pro can perform 2 for you for a large variety of libraries such as MSVC libc from earliest to latest version's, and if it somehow couldn't you could always make your own FLIRT signatures for it to identify. Of course, if you could make your own FLIRT signatures, it would be just as easy to write your own code to do the indentification automatically.
In general, I would suggest you try to find what exactly you want to hook by properly reverse engineering the binary, instead of hoping you could somehow magically hook something which you don't know exactly about (there can be a lot of ways strcmp can be implemented, different per implementation (both library and compiler), and sometimes even the user may use a different version for some reason(rare)).