A common misconception about Notaries is that they are always required to witness a client signing the document in the Notary's presence. But that is not always true. Many states allow Notaries to execute acknowledgments on documents that already have been signed. However, a signer does need to always sign a document requiring a jurat in the Notary's presence.
When a pre-signed document is presented to you for acknowledgment, it's a good idea not to take for granted that the signature belongs to your client. There's nothing to prevent you from asking, "Is this your freely made signature?" After all, it is part of your duties as a Notary to verify that your client signed the document willingly. This simple question might also spook someone trying to commit fraud just enough to have second thoughts. It will also help you determine your client's willingness and awareness.