Accessible at https://www.sign-web.app/explorer the explorer is a straight forward way to search if a file has already been certified or if a certification sent to you is valid.
VERIFY BY PROOF OF CERTIFICATION
You can verify a certification simply by selecting it's Proof Of Certification after clicking on the "Select or Drop a Proof Of Certification" button:
The Proof Of Certification is a PDF that looks like the following:
This PDF includes the details of the Certification as well as some metadata allowing the SIGN explorer to automatically read this information and retrieve the associated transaction from the Waves Blockchain.
If the transaction exists, the certificate will be displayed in the SIGN explorer as follow:
VERIFY BY HASHING A FILE
You also have the option to hash any file, using the "Select or Drop a File to hash" and it will retrieve all the certifications, if any, that are included with this exact file hash:
Once the file is selected, its hash will be added in the Hash field and by clicking the "SEARCH" button you will start retrieving the data.
VERIFY BY DATA
Similarly you can choose to search directly by Transaction ID, File Hash, Message ID or reference as well as combining some of them for more accurate results.
Direct link to a Certification in the explorer looks like the following:
If the certificate have been revoked by its creator, you will see the "revoked" mention on the top right corner: