How can I verify the authenticity of the digital signature?

You can verify the digital signature through the "Validation" option offered by Endesa.
To do this:

You have to have first stored the corresponding electronic bills on your PC by clicking on the "@" icon in the "billing history".

Then click on the validation icon. A page will open and using the "Browse" button, you choose the electronic bill you want to validate.

Once the reference appears in the window, click the "Validate" button and you will get the information you want: signature owner, issuer information and the signature's period of validity.