Birbal, Can you identify Guest

Once Birbal was invited for dinner by a rich man. When Birbal reached there, he found himself in a large crowd. The host greeted him warmly and took him inside. Birbal said, "I did not know that there will be so many guests in this gathering." The host replied politely, "They are not guests, Sir. They are my employees except one who is the only other guest here besides you. Could you tell who is that other one guest here?"

"Maybe, I could. Tell them a joke, and I will observe them." The rich man told the joke and everybody laughed uproariously. Perhaps this was the worst joke Birbal had ever heard in his life. Now the rich man asked Birbal, "I have told the joke, now you tell me who is the other guest here?" Birbal pointed out towards a man and said, "He is that other guest."  The rich man was very surprised hearing this that how could he recognize the other guest. He said to him, "You are right Birbal, but how did you recognize him?"

Birbal said, "Because only employees can laugh on such a joke. He was the only person who did not even smile on your joke, so I immediately recognized him as the other guest."