how can we be sure that be autistic or not ? can we tested as much as possible with private organisation or doctors ... do you have any suggestion near the Milton Keynes

I want to help my friends because he realized that his daughter might be autistic. His daughter's schools suggested him that if you go to GP you spend almost 2 years to understand what is the main problem so you can go to private organization to help understand what's going on ? Do we have to spend almost 2 years to figure out all the things . Is there any chance to find  someone to report quicker and understand all the things. Please if you know some organization or what should we do please leave comment and share with me. Thanks ....