What circuits don't work with GFI outlets?

I had to run a new cable when I needed one on a multiwire circuit. I guess shared neutrals are out. I recently was asked to add one on a circuit though, that wasn't a shared neutral. How can you test a circuit to see if it'll work with a GFI?

I guess the easiest way to test for compatability would be to use a short GFI extension cord.
Joe

Thanks joe.