If you have reason to believe that a signer is trying to commit a fraud by using an obviously faked ID or bogus document, or is being coerced by another party, you should stop the notarization, note the circumstances in your journal and contact your local law enforcement authorities immediately. That way, the police may be able to stop the criminal from finding another Notary who might not be as observant as you.
However, if the suspicious signer threatens you verbally or physically, don't put yourself at risk. Proceed with the notarization and contact the police as soon as they have gone. Regardless of the circumstances, you should be prepared to turn over any evidence -- such as copies of your journal entry -- to the authorities and give a statement about the incident.