How can I know when a file was signed?
You can prove that a document was signed after a certain date by including data that was not known to anyone before that date, such as stock market data.

You cannot prove that a document was signed before a certain date by purely cryptographic means. Information doesn't go stale, so when you show a signature, it could have been signed at any time.

You can prove that a document was signed before a certain date by non-cryptographic means. Showing the signed document to someone (a trusted third party) is an easy way to do that: if you trust that entity to vouch for having seen the signature at a certain date, then the signature has to be older. The third party doesn't need to see the document, only its signature (containing a hash of the document); if you need to prove that the document was signed, you can reveal the document later.

None of this is what you need, though. You would like to prove that a document was not signed before a certain date — the date when your private key was stolen. The thief could create a signature and pretend that the signature had been made a long time ago (but they will of course not be able to prove it: it would be your word against his).

One way to accomplish what you want is to require that any valid signature be “notarized”: the recipient must be instructed never to trust your signature alone as genuine, only your signature validated by a trusted third party (the notary), which would demonstrate the existence of the signature at the date it was seed by the notary.