The Naught Vaught Test
Over a language L, if a theory T has an infinite model and is κ-categorical for some κ≥∣L∣, then T is complete
Take T satisfying the preconditions, and assume for contradiction that T is not complete.
Then exists some L-sentence ϕ for which both T⊭ϕ and T⊭¬ϕ. Hence both T∪{ϕ} and T∪{¬ϕ} are satisfiable.
By a theorem I am unable to locate (part of the Henkin construction?), since T has an infinite model and since κ≥∣L∣, then we are able to construct models M⊨T∪{¬ϕ} and N⊨T∪{ϕ} of size κ.
Since these models satisfy different setences, then they are not isomorphic, so T is not κ-categorical; contradiction.