Watch for Signs of Boredom

If someone likes you, they will enjoy being with you. So if the person you're with shows signs of being bored, that's a warning sign. Be sure you don't confuse sleepiness with boredom! Lots of people get too little sleep and end up sleepy when they're in public. Maybe make a joke about the person being sleepy, and see how they respond, if they start having their attention drift.