using VehiclePhysics;
VehicleBase.WheelState ws = vehicle.wheelState[wheelIndex];
float forwardForce = ws.tireForce.y;
float sidewaysForce = ws.tireForce.x;
wheelIndex is the index of the wheel. Wheels are indexed left to right, then front to rear. In standard 4-wheeled cars front wheels are 0, 1 (left, right) and rear wheels are 2, 3.
The reference of the class VehicleBase.WheelState with all the exposed values is here:
https://vehiclephysics.com/advanced/vehiclebase-reference/#nested-classes