It's hard to respond to your post because you don't seem to know the difference between "naughtiness" and autistic behaviour, which usually parents with autistic kids are pretty well on the ball with. Your post comes across as goady which I'm sure wasn't your intention. The views that you are expressing about autism are decades out of date.
I've asked a lot of questions but they are rhetorical, designed for you to work things out for yourself. Apologies in advance if it comes across as quite blunt in places, it's not meant to be, but it's not the usual type of questions that a parent of an autistic child would ask.
How easy to tell if your childs bad behaviour is due to autism or just plain naughty
It's really hard to believe a parent of an autistic child would ask that. What sort of "bad behaviour" or "naughtiness"
If it's sensory overload then no, it's not naughtiness. If it's anything to do with how his autism affects him, then no, it's not naughtiness, he's distressed and his behaviour is letting you know he has a problem but not the ability to help himself.
Don't punish him for his disability and don't let anyone else do that either.
no one in my family beleives in autism
Do they have the medical qualifications and the expertise to diagnose autism?
If not, then why on earth are you listening to their uninformed opinions?
Change the word Autism in your sentence to Epilepsy or Diabetes and see how ridiculous it looks.Would you give credence to the opinions of anyone who said they don't believe in Epilepsy or Diabetes? Then why are you listening to their opinions about Autism?
He rarely misbehaves with me but put him with his grandparents dad or uncle and his behaviour is off the scale
Do you treat him as an autistic child and they don't? What actually causes that?
I actually think he is being naughty on purpose as he seems to love the reactions he gets
Maybe he's sensory-seeking?
Maybe the professionals got it wrong and I am not parenting him correctly
As if the professionals can't tell the difference between autism and naughtiness?
Have you any idea how difficult it can be to get a diagnosis, how much work goes into it often by a multidisciplinary team of medical professionals?
To be diagnosed, this is from the NAS website
"Is it autism?
The characteristics of autism vary from one person to another, but in order for a diagnosis to be made, a person will usually be assessed as having had persistent difficulties with social communication and social interaction and restricted and repetitive patterns of behaviours, activities or interests since early childhood, to the extent that these "limit and impair everyday functioning" "
What does his diagnosis say about his strengths and weaknesses, how does it suggest he can be helped at home as well as at school?
Is he getting all the help he needs?
I looked through some of your previous posts and would say that from your descriptions of both your kids' behaviour, your knowledge of autism seems to be fairly scant, either that or you just don't get it across in your posts. Based on what you've written, I'd suggest you really need to read up on the condition and all its co-morbids and its differences in presentation between boys and girls, particularly the ability (called masking) to hold it all together at school all day then let all the bottled-up overwhelm go when they get home. That alone is a big red flag.
Some resources
www.autism.org.uk/
Ross Greene's The Explosive Child and his website Lives in the Balance
See websites and youtube for SPD, PDA, Executive Function and the differences in the presentation of autism between boys and girls.
The more you can understand your child's condition, the more you can help them to achieve their potential. You need to be their advocate when they can't cope.
Above all, stop listening to opinions from people who know nothing about autism.