There is no simple formula to determine whether a therapist is competent in what they do. Credentials, diplomas, certifications and licenses may be impressive, but still may not indicate or guarantee that a therapist is competent or ethical when helping people. At best, credentials only inform you of the extent to which the therapist has pursued his or her academic education and training in their profession.
Membership in a recognized professional organization, like the National Association of Social Workers or the American Psychological Association for example, may be important. Associations like these hold the practitioner accountable for following their organization's professional code of ethics.
Also, the Colorado Department of Regulatory Agencies (DORA) Mental Health Grievance Board keeps a public record of complaints filed against psychotherapists. Anyone can call this department at (303) 894-7766 to learn if complaints have ever been filed against a licensed or unlicensed psychotherapist. This information is also available online: https://www.doradls.state.co.us/alison.php.