The Prefect 3.x documentation does not provide a direct method to determine the exact trigger source (manual or scheduled) of a flow run from within the flow itself. However, you might explore using Prefect's event-driven automation to track and log events related to flow runs. This could potentially be configured to differentiate between manual and scheduled triggers based on the metadata or naming conventions you establish for your deployments.
For more detailed configuration and event handling, you might want to explore Prefect's automation and event documentation further:
Prefect Automation and Events.
If you require more specific functionality, consider reaching out to Prefect support or exploring their community forums for custom solutions.