A Google employee falsely claimed a chatbot had sentience. How do we know he was wrong?

Here's a big woolly question: How do we know when a machine is sentient? A few days ago, a Google software engineer and artificial intelligence (AI) researcher claimed the tech company's latest system for generating chatbots was exactly that: sentient. Since then, leading AI researchers have dismissed this claim, saying the AI was essentially faking it. Google's chatbot system isn't sentient, but one of its eventual successors may be. When -- or if the time comes, how -- will we know?