JTB World’s DigSigStamp app allows you to specify a block that will change its display when plotted to indicate whether the drawing had a valid digital signature at the time of plotting.
DigSigStamp has now been updated for AutoCAD 2017.
The following are just examples of how the blocks can look like. The design of them is completely up to you. Digitally Signed example. This drawing has a valid digital certificate.
Invalid. This drawing does not have a valid digital certificate.
Unverified example. Digital signature verification application is not enabled.