When you open a PDF document that has a signature, Acrobat will attempt to verify the signature based on trusted certificates.
If the signature(s) cannot be verified, you will see an indication that they need to be verified, usually a question mark symbol. To verify a signature, click on the signature and the Signature Validation Status dialog should open. Click on the Signature Properties button to open the properties dialog. Here you can verify the signature(s). For assistance on how to do this use Acrobat Help.