CharacterController:

bool isGrounded =
    controller.isGrounded;

CheckSphere:

bool isGrounded =
    Physics.CheckSphere(
        groundCheck.position,
        groundRadius,
        groundLayer
    );

Raycast:

bool isGrounded =
    Physics.Raycast(
        transform.position,
        Vector3.down,
        groundDistance,
        groundLayer
    );