Verify an ISO 9001 certificate

You can ask them their certificate or get a copy. Check the name of the certification body and verify if its name is in any list of an accreditation body recognized by the International Accreditation Forum (IAF). If that certification body is working with an accreditation body recognized by the IAF you can contact them,and confirm if their certificate is valid or not.

The following material will provide you more information about solving doubts around a certificate validity:

• How to know whether ISO 9001 certificate is valid? – https://advisera.com/9001academy/blog/2018/05/23/how-to-know-whether-iso-9001-certificate-is-valid/
• Free webinar – ISO 9001:2015 clause 4 – Context of the organization, interested parties, and scope – https://advisera.com/9001academy/webinar/iso-90012015-clause-4-context-of-the-organization-interested-parties-and-scope-free-webinar-on-demand/
• Enroll for free course – ISO 9001:2015 Foundations Course – https://advisera.com/training/iso-9001-foundations-course/
• Book – Discover ISO 9001:2015 Through Practical Examples – https://advisera.com/books/discover-iso-9001-2015-through-practical-examples/