Don't fiddle with the headstock adjustment unless the machine has been crashed. I've set up 6 lathes of my own many times, and never touched a headstock bolt on a single one of them yet
The typical lathe can easily be aligned by adjusting the levelling feet at either end of the bed. If the taper of the chucked part is smaller towards the tailstock end, this means the bed is twisted toward the back of the machine, causing the carriage to approach at an angle to the headstock. Adjust the levelling screw downwards (or shim upwards on the floor) on the backside of the bed, at the tailstock end.
This is a delicate miniscule adjustment to make. You think your lathe bed is rigid? It's as flexible as playdoh
This has nothing to do with adjusting the tailstock setover. The test bar that Sid8ball described is the proper method to detect the alignment of the bed with the spindle. You need a good quality chuck with an accurate set of jaws to grip the stock along the entire jaw length, otherwise a wobble factor will set in when you take the test cut, rendering it useless. I usually neck the stock down leaving an inch at the end and an inch of width near the chuck jaws to take the test cut with. A very sharp tool with plenty of positive rake is necessary to prevent tool deflection from messing with your head on this.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)