When should the clutch be replaced?

I haven't driven my stang in about a year, but it seemed to me at the time that it was loosing power over time. I don't know if this is just a mind trick since I have gotten used to it or not. I was wondering if a slipping clutch would cause this sort of perceived problem? Also what are some signs of a clutch that is ready to be replaced? Oh and my car has about 120K on it....

Thanks in advance.....