I would say that typically depends on which disorder we are talking about. Different disorders have different start points and early symptoms. We often call the early signs "prodromal symptoms" or a prodrome. For schizophrenia, we might see changes in mood, behavior, and sleep.
Great answer, @Peter B. On that same note, Chapman in 1978 described 5 patterns of prodromal disturbances for schizophrenia: disturbances in attention, perception, speech production, motor functions, and thought block. Different studies will have different structures and conceptualizations but these are common signs or indications for consideration.
I would say that typically depends on which disorder we are talking about. Different disorders have different start points and early symptoms. We often call the early signs "prodromal symptoms" or a prodrome. For schizophrenia, we might see changes in mood, behavior, and sleep.