Caster is a measure of the angle between the steering axis and vertical, as viewed from the side of the vehicle when the wheels are in the straight ahead position. Stated another way, it is the tilting of the front steering axis either forward or backward from the vertical. A backward tilt is said to be positive (+) and a forward tilt is said to be negative (-).
Although is it measured using a special instrument, it can usually be seen by observing the location of the upper and lower control arm ball joints. A line drawn through the center of these 2 points represents the steering axis. When looking straight downward from the top of the upper control arm you can see if the ball joints are not aligned, indicating that the caster angle is more or less than 0 degrees. If vehicle has positive caster, the lower ball joint would be located ahead of the upper ball joint center line. If the vehicle has negative caster, the lower ball joint would be located behind the upper joint center line.
On 2wd trucks, the caster may be adjusted by changing placement of shims on the 2 upper control arm pivot shaft-to-frame bolts. On 4wd trucks, the caster is adjusted by turning the adjustment cams. To increase positive caster on a 4wd vehicle, move the front cam lobe inboard and the rear cam lobe outboard.