You can get an independent private assessment from one of the dyslexia centres or a specialist educational psychologist - check they have experience working with and assessing children with dyslexia [ this is vital]
Or you can ask the school or college for a refer for an assessment which is free but you may have to wait.
If you contact the Dyslexic Institute or British Dyslexia Association they will let you know what is in your area.
Good luck