I follow my heart rate--our machines have the built in pulse monitors that SOMETIMES work...
And when they do, that's what I go by. I find that I don't feel like quitting if I stay in the range for my pulse. Also, there are TV monitors, so I look at them from time to time. But mostly it's that I don't get tired out and want to quit--if I stay in the range, I could go all day.
I also like to burn over 100 calories per workout--today I was up to 182 on the elliptical (30 minutes). So that means a nice entry in my chart!