Cylinder check

How would I go about checking to see if I lost a cylinder. I onced cruised at 147 with my 944 but can no longer hit 130. I could always go to a shop and get it checked but I dont know much about cars and tend to get ripped off all the time when I get my porsche fixed. I've also noticed I cant even tell when I throw it into 5th gear, it even seems weaker when I do. How would I go about checking to see fi I lost a cylinder?