CharacterController

bool enSuelo =
    controller.isGrounded;

CheckSphere

bool enSuelo =
    Physics.CheckSphere(
        groundCheck.position,
        radio,
        layerSuelo
    );

Raycast

bool enSuelo =
    Physics.Raycast(
        transform.position,
        Vector3.down,
        distancia,
        layerSuelo
    );

Resumen rápido

controller.isGrounded
Physics.CheckSphere(...)
Physics.Raycast(...)