I use the tachometer to see how far I am in the redline, I never use the red blinky light. It varies because I will also shift when I feel the car has reached its max in that gear.
I follow the sound to determine shift points, though sometimes I am forced to go by eye, whoever that voted by eye solely needs to get readjusted to listening for the shift point, you'll be a lot better.
I listen to my engine to know when to shift, which throws me off if I do a hybrid race like one I had in FM4 a few weeks back. It must've been the most silent race in motorsport history, with only the occasional tire squeal. Bounce the needle off the red line and not even know it until I look down at the tach.
A bit of both. If the car's engine has an unusual powerband I'll shift by eye, although I don't stare at the tach when upshifting, usually just a quick glance or using peripheral vision. Most of the time I shift by a combination of hearing and peripheral vision.
I do it by ear mostly - though a glance at the tach occasionally does help. However one does also tend to shift due to circumstances - 'dropping a gear' for instance for a quick burst of speed, or down-shifting deliberately to use a lower gear for better traction.