Neuropsych (psychologist) can dx anything in the realm of the DSM (list of disorders related to mental health issues, disorders of cognition, ASDs, and learning disabilities). They rely heavily on a customized battery of standardized tests they administer. They will also conduct a thorough interview to asses dx level of functioning, significant history, and issues of concern. They will make up an evaluative report with results and recommendations.
A developmental ped is an MD who had a thorough understanding of both the DSM and the broader range of medical diagnoses, especially those effective a child developmentally and/or behaviorally. Their eval will usually consist of a thorough medical exam (possibly even bloodwork or genetic testing), review of medical history/records, a thorough patient/parent interview and possibly parent/teacher behavioral surveys. They would not usually perform standardized testing, but would refer a patient to a neuropsych if they saw a need to investigate a specific issue further.
There is no need for a neuropsych eval to determine things like depression, behavior disorders, mood issues, anxiety problems, and typical ADHD. However it can be invaluable to help untangle confounding or cocurrent problems such as ADHD vs. ASDs vs. expressive language delay (for example).
