you would put this command in an expression flow on all the nodes where the knowledge base gets checked. If it doesn't you can redirect the flow.

turn.KnowledgeAgent.Responded

is the opposite , if it responded. In some versions or setups, you might be using something like

{{turn.KnowledgeAgent.answer}}

. If

turn.KnowledgeAgent.answer

is empty or undefined, it implies that the Knowledge Base didn't have an answer.