Slipon's are mounted to the stock/existing system. Typically they're clamped on a few inches after the midpipe.
Full system's replace the stock system (or most of it, anyway) and have a better-designed curve from where the rear pipe turns back to meet with the front pipe. Stock system has a very tight bend, full system usually not so tight & more visually appealing.
Post a picture and it'll be easier to show you.

 

If the headers you're looking at are EXACTLY the same as you're stock ones then it's stock. If the headers have fewer mounts and springs holding stuff together then it's probably a full system. Not all full systems have springs but most do, stock does not.