The Statement of Accomplishment issued by the class.stanford.edu Team is signed by a gpg key so that it can be validated independently by anyone who has the class.stanford.edu public key. For independent verification OpenEdX uses what is called a "detached signature" meaning that the signature for the Statement of Accomplishment is contained in a separate file with a ".sig" extension.

To complete the verification procedure you will need the following three files:

The official class.stanford.edu Team public key.
The class.stanford.edu Statement of Accomplishment belonging to Roberto Elias Alonso Sanchez (you should already have this file).
The signature file for Roberto Elias Alonso Sanchez's Statement of Accomplishment

Ensure you have the pdf, the ".sig" signature file and the class.stanford.edu public key copied to a single directory before you begin.

Import the class.stanford.edu public key on OSX using GPG Keychain

Download and install gpgtools.
After installation, GPG Keychain Access will prompt for a new personal keypair.
Click the import icon to import the public key file, class_stanford_edu.pub.
CTRL-click the class.stanford.edu public key and select "sign" to validate it.
Confirm that the class.stanford.edu public key is listed in the GPG Keychain Access window and it has Short ID 3E862F0D.

Import the class.stanford.edu public key on Microsoft Windows using gpg4win

Download and install the full version of gpg4win.
After the installation is complete, launch Kleapatra from the start menu.
Create a new personal certificate if don't have one listed under "My Certificates".

When prompted, select "Create personal OpenPGP key pair."

new certificate

Import the class.stanford.edu public key file, class_stanford_edu.pub for class.stanford.edu by clicking on the "Import Certificate" icon.
Certify the class.stanford.edu public key by highlighting the class.stanford.edu public key/certificate and selecting "Certify Certificate" under the "Certificates" menu.
Make sure that the class.stanford.edu public key/certificate is listed under the "Trusted Certificates" tab and verify it has the Key-ID 3E862F0D.

Verify the class.stanford.edu Statement of Accomplishment using gpg on both OSX and Windows

Open up a new terminal on OSX or a command window on Windows
Go into the directory where the .sig and .pdf files are located
Run the following command to verify the fingerprint of the class.stanford.edu key

gpg --verify Certificate.pdf.sig Certificate.pdf

Confirm that "Good signature from "class.stanford.edu Team <techsupport@class.stanford.edu>" is reported by gpg