1. Go to the tenant's admin console by typing https://<tenantFQDN>/admin/login
2. From the Admin Console go to Settings > Apple/iOS Certificates
3. Take note of the last 8 or so characters of the MDM Certificate Name:
 

4. Go to https://identity.apple.com/pushcert and log into the Push Certificates Portal
5. From the list of certificates identify each certificate by clicking the "Certificates Details" button:

4. If no matching certificate is shown verify which Apple Account was used to initially create the certificate.