Traction Circle Limits

How are the outer limits of the traction circle calculated? How does a driver know how how far away from the edge they are?

It can't be calculated with accuracy, just assessed from tyre loadings and characteristics.

From the driver's viewpoint, it can sometimes be felt as an increase in yaw from the rear drifting a little wider and reduction in lateral force, and at the front the steering will tend to load up to the point of maximum 'grip' and then ease off as the tyre slip angle increases and grip is reduced - if you've gone past that point, you can sometimes feel the steering loading up slightly as the load is reduced on the exit.

The tricky part is that as the tyre gets more 'race' oriented the transition area gets more and more concentrated and the tyres can break away more quickly.