The testes should be about the same size. If a baby's testicle is firm or very red, this suggests a problem. If it is firm or does not move, this is a sign. If one testicle is "missing," this is a sign.
With testicular torsion, the scrotum will be very tender, red, and either swollen or firm. The baby will be very uncomfortable.
Testicular torsion is a medical emergency. Since all blood for the testis comes through the spermatic cord, a twisted chord cuts the blood supply. The testis will shrink ("atrophy") if blood flow isn’t quickly restored.