I don’t know of anyone who actually measures it. Like Gary said, it’s all relative.
Your kart comes with some built in caster. Then you’ve got the caster pills which give you several degrees of adjustment either way. You always start them at neutral, and then adjust from there based on what the kart needs. There’s no reason to have the actual hard number.
If you know your kart has 12* of built in caster and you have 3* worth of pills in the front end, you know that full caster is 15* and minimum caster is 9*. Half caster in is 13.5*, half caster out is 10.5*. If you have 3* pills in the top and bottom of the kingpin, those values double.