By native google, Dialogflow doesn't support bot to human handoff or provide any Web interface to achieve this, but in dialogflow normally the action “input.unknown” used to hand off the conversation to a Human. The “input.unknown” action is built into Dialogflow and used with the default fallback intent. When none of the intents is matched, the default fallback intent is triggered and action associated with it is added in the response also you need to write a logic what bot should do after that.
But to make things easier you can integrate dialogflow with any third-party tool like Kommunicate. As they provide pre-built chat widget user interface and bot to human handoff can be enabled on a click of a button, Please click here for more detailed information
PS: I work for Kommunicate