how reliable are adult diagnosis of aspergers and autim

is it possible to get an accurate diagnosis of aspergers or autism as an adult and if so how do they determine diagnostic theories to support diagnosis

[spacebast]

You're right, it depends on the underlying model for autism sprectre disorders (ASD).

In Germany, the diagnosis of ASD is at least task of psychatrists. Psychologists do some basic tests; eye tracking experiments can give additional hints leading to ASD; MRI is made to exclude (visible) neurological issues.
At least, an experienced psychatrist have an interview with the subject/patient, if available with its relatives about behaviours, biography (e.g. about beginning of speech)
To differentiate Asperger and HFA:
If you imagine a continuum of ASD beginning with early childhood autism...:

Early childhood autism (formerly "CANNER") --> Asperger --> High functioning autism HFA)

you have an increase of (measurable!) intelligence
you have an increase of social abilities/compatibility
According to this, you can differentiate Asperger and HFA with common test of intelligence. So, if the basic tests, the behavioural and biographical analysis lead to ASD, the diagnosis of Asperger is made with average intelligence and of HFA with intelligence above average.

About the accuracy of diagnosis: *** is a complex disorder - so some lacks in accuracy of diagnosis should be accepted.

I'm not mother tongue, so I hope my explanations are understandable.