Ettina, it will also be hard as adults with autism can exhibit delusions and/or hallucinations in a transient manner when highly overwhelmed, i.e., days to weeks. This could lead to a misdiagnosis of comorbid schizophrenia if the clinicians don't get the duration of such down accurately. To complicate matters, comorbid depression is common among adults with autism, which will bring about anhedonia, so a lack of pleasure in interests can look like the individual is developing schizophrenia with the aforementioned transient delusions and/or hallucinations.
Symptom onset of both, as you say, is the key to determining such. Childhood schizophrenia is hard to distinguish from autism, albeit autism will tend to have that narrowly defined interest that brings the individual pleasure, where it'll be mostly absent in childhood schizophrenia (there'll be a lack of pleasure in interests with the latter). Complicating matters, childhood schizophrenia has a strong correlation with a cormorbid ASD.
Obvious autism, with the positive symptoms of schizophrenia starting up around the right time, especially proceeding stressful life events, will be the typical comorbid presentation as you describe. At least the quite evident one.
I also have that fun paradoxical laughter/pseudobulbar affect. Which can be funny in a way.