Short answer: Ask your plan for its medical-necessity criteria, the reason for any denial, and its comparative analysis of treatment limits. If they show stricter rules for mental health care, you can appeal and file a complaint with the U.S. Department of Labor.
Federal rules give you a right to information: a plan must make available, on request, the criteria for medical necessity determinations made under a group health plan with respect to mental health or substance use disorder benefits, and the reason for any denial of payment. Since the Consolidated Appropriations Act, 2021, plans must also perform and document a comparative analysis of their non-quantitative treatment limitations, such as prior authorization or network standards, and provide it when asked.
A practical path is to request those documents, then file your plan’s internal appeal if a service was denied. You can also contact the Department of Labor’s Employee Benefits Security Administration for help with a job-based plan, or your state insurance department if the plan is state-regulated. Keep copies of every denial and the plan’s responses in case the dispute escalates.