Pelo menos nos EUA, nao se usa "to look in" para dizer "to watch TV".  Usa-se, porem, para dizer visitar, dar uma olhada em, etc. pessoas que estamos cuidando, pessoas por quem estamos responsaveis.

Mary drove by the hospital and looked in on her father.

George, look in on the children.  They are very quiet and I am worried that they are misbehaving.

You can study in here.  I'll be back in an hour or so to look in on you, to see if you need anything.