Please note: Some certificates and Approvals are now being issued without any signature of an authorised representative of the certification company. The certification company then usually offers the possibility of checking the validity of certificates in real-time online. This is often also indicated in the certification document itself.
A DNV Type Approval Certificate for example indicates on the last page:
To check the validity of this certificate, please look it up in https://approvalfinder.dnv.com.