In the modal mental health outpatient at VA, it seems the main issue would be with the forest of commonly presenting comorbidities/issues like clinical depression, PTSD, sleep apnea, substance abuse, personality disorders, symptom overreporting (either for $$$ or a dopamine push). Any thorough evaluative process would have to rule in/out all of these other contributors to putative ADHD symptoms and likely extend over several sessions.
At my VA, ADHD 'evals' basically break down into the following scenario:
Veteran wants access to a stimulant to 'treat his ADHD.' Psychiatry won't write script until psychology 'assesses' and diagnoses ADHD (thus transferring responsibility for making the dx and decision to prescribe stimulant from psychiatrist to psychologist). Psychology almost always says 'yes' to ADHD dx...vet gets meds. Rinse and repeat.