I have this one white short vertical line inside my cheek.its in the back of my mouth so i can barley see it, but its right beside my molars. ?

Hard to say: Can you elaborate, do you have pain or swelling or other symptoms associated with this line ? Is it flat or raised,? How long have you had it. Some of patients have lines next to there teeth , like a demarcation line that suggest pressure from the check against the teeth, some times you can see it against the tongue as well. To be sure you need to see a dentist..

Cannot say: May be a scar from a previous injury, the start of a Lichen Planus infection, an area of irritation, and so many other possibilities. Please have your Dentist examine the area. Once evaluated, the proper course of treatment ( which may be just watchful waiting) can be administered.

Most likely: I would guess that it might be what is know as linea alba and is fairly common and innocuous. Google linea alba of the mouth and see if that is what it looks like. However, it could be any number of other things and should be seen by your dentist for a definitive diagnosis.