A diagnosis of SM is usually provided after team assessment involving a Speech Pathologist, Paediatrician, Psychologist and/or Psychiatrist ASHA - http://www.asha.org/public/speech/disorders/selectivemutism/
You can approach your local General Practitioner for a referral to these services or look for services in the area that show on their website that they have experience in the area of SM.
A diagnosis of SM is typically made when assessment has demonstrated that the individual meets the following criteria:
Consistent failure to speak in specific social situations (in which there is an expectation for speaking, such as at school) despite speaking in other situations.
Not speaking interferes with school or work, or with social communication.
Lasts at least 1 month (not limited to the first month of school).
Failure to speak is not due to a lack of knowledge of, or comfort, with the spoken language required in the social situation not due to a communication disorder (e.g., stuttering).
It does not occur exclusively during the course of a pervasive developmental disorder (PPD), schizophrenia, or other psychotic disorder.
The Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM–5; American Psychiatric Association, 2013). - See more at: http://www.asha.org/public/speech/disorders/selectivemutism/#sthash.LMIUKaQP.dpuf
It is essential that the assessment includes a detailed case history/parental interview, information gathering from school and 1:1 assessment with the child for hearing, speech and language concerns.
According to some professionals it is not possible to have a co-diagnosis of SM and Autism Spectrum Disorder (ASD) due to the criteria of the DSM. However, leading specialists in the area would argue that a diagnosis is possible – SM would be the secondary diagnosis and ASD the primary e.g. Dr Shipon Blum, Maggie Johnson and there is research out there to indicate that there is a high prevalence of SM within the ASD population.
Where SM is suspected when a diagnosis of ASD has already been provided it is vital to identify the cause of the absence of talking. If the individual is not experiencing discomfort due their silence it may be that they are not speaking due to difficulties understanding social interactions (how to interpret others behaviour and how to predict and respond)or communication difficulties inherent with their diagnosis of ASD. As such intervention should focus more on explicit instructions for social interaction/communication and then building comfort in social situations rather than the latter first and foremost. Similarly it is important to acknowledge that speaking anxiety can occur in addition to ASD and thus should be treated accordingly.