Write a letter/email to the owner and send it via the agent. The agent would then have a legal responsibility to inform the owners and the agent can become liable for damages as a result of failing to fulfil the agent's duties if they do not inform the owner of a significant change regarding the leasing agreement. You should mention this fact in the email.
Note that oral changes to a written agreement may not be acceptable to a court/tribunal. You'd likely need other strong evidence/behaviour indicating the presence and acceptance of a change.
You can pay for a title search but the title may have an old address on it and unless it is close to where you live to confirm their location it may not help you.
Most times the local council will not help, but you can check electoral rolls (Federal, State, maybe council?).
