Caster is harder to see with the naked eye. Imagine a line running perpendicular from the ground up through the top strut mount (red arrow). Now imagine a second line running from the lower ball joint up through the top strut mount. If the second line is tilted forward toward the front of the car, you have positive caster (yellow arrow). If it tilts toward the rear, you have negative caster (blue arrow), which you should never have.