Since wrote:
The symptoms? Particularly hallucinations.
Schizoid personality disorder doesn't involve hallucinations. It is basically an extreme personality type that involves very strong introversion and a tendency towards either lack of emotion or lack of emotional expression.
You can tell the difference because an autistic person can be extroverted, and because he will have much stronger social and language delay, and will have some sort of repetitive activity or intense interest that the schizoid will not have. Actually, if you see someone with language delay, special interests, or stims, you can almost certainly say that if he has either one, it's autism.
If someone with just schizoid personality disorder has a social skills deficit, it will be purely from lack of practice. With autism, it is innate.
You cannot technically be diagnosed with both, but I believe it is possible for an autistic person to also have schizoid traits.