how do i know if my clutch is slipping

I have a 94 Honda civic 2dr ex when I take off my car will get up about 3500 rpms then go down then pick up speed after the clutch has been released then also going down the road if I give my car to much gas it will get up to high rpms then go down then pick up speed is this a sign of my clutch slipping if not then what could this be