If the current document has signatures a bar will appear above the page view after the document opening. It allows you to open the Signatures panel.
Using the Signatures panel you can inspect the document signature and its certificate.
This feature is available for PDF documents only. You need to install at least version 0.51 of Poppler to get most signature information, version 0.68 to get signature location/reason information, and version 0.73 to get signature certificate information.