With the wheel turned and better access, I just stuck a BMF screwdriver or pry bar to move the control arm out slightly to install shims.
On my bubble gauge if I'm on the left side, I turn the wheel to the left 20°, zero the gauge and turn it 40° or 20° to the right and the caster number will show.
It's pretty easy to tell if you have positive or negative caster just by looking at the wheel when turned. If you're on the left, turn the wheel to the left. For positive, you can see the top of the tire is leaning back. Negative, iti leaning to the front
One thing great about getting older. A life in prison sentence is less of a deterrence