Jitter can be differentiated from cable loss because jitter is not really changed over long lengths of cable whereas other signal parameters such as rise-time, fall-time and amplitude are severely impacted by long runs of cable. If you see an eye diagram that has the eyes "closed" but the rise/fall time and amplitude are within normal ranges then you most likely have a lot of jitter. The opposite situation is if you have a closed eye diagram with normal jitter values but stretched out rise/fall times and reduced amplitude, this is most likely the result of long cable lengths.