Transmission skipping

I have a 2001 Alero. It just started to jump when shifting from 1 gear to another. It does not do this in the morning when I first start out to work but it quickly happens afterwards. I park at night on a rather steep inclined driveway. I would check the transmission fluid but do not know how to go about it since I belive it is a sealed system. Do this sound like a reasonable cause or is it more involved?