Raycast frontal

Ray rayo = new Ray(
    camara.transform.position,
    camara.transform.forward
);

if (Physics.Raycast(
    rayo,
    out RaycastHit hit,
    distancia,
    layerMask))
{
    Debug.Log(hit.collider.name);
}

Desde centro de pantalla

Ray rayo =
    camara.ViewportPointToRay(
        new Vector3(
            0.5f,
            0.5f,
            0f
        )
    );

Resumen rápido

Camera.ViewportPointToRay(...)
Physics.Raycast(...)