How should you know? Actually an interesting problem: A1. You say you don't experience emotions. A2. What you call emotions is what you see others do when they say they are acting emotionally. You don't think you are this way. A3. Your doctor says you're right. Or: B1. You say you experience emotions. B2. Others say you are wrong.
The only way to really get outside this kind of introverted bubble is to ask people or confide in them. This leads to conversations like whether an emotion like happiness is an actual thing or not that they can identify and actually feel. In the sense that they are actually sitting there and willing to state "I feel happy", it's different from yesterday. Most people are hence why I diagnose myself in line with this article.
Or, another way is people generally have an answer to the "Tell me about a time you were very happy"... a lot of people can reel something off really easily here. Sometimes it's like a wedding and you think "you could be saying that because it's accepted as a good answer" but other people say a certain day at the beach where something really specific happens and that lends credence to the idea they feel this emotion and I don't.