Just to clarify the jargon here? You don’t mean signs. You mean physical findings. Anything observable - including behavior - is considered a sign, not a symptom. And schizophrenia has any number of well-described “signs” including a bunch of speech peculiarities, flat affect, mannerisms, etc etc etc.
Considering that somebody who’s psychotic is unlikely to blurt out “I’m having delusions,” schizophrenia is probably diagnosed more on signs than most mental disorders.
Not that that changes anything, but just for the sake of accuracy.