There are a few ways to tell if someone is texting someone else. If you are in a conversation with the person and they start to type on their phone without looking up, they are probably texting someone else. Another way to tell is if the person's eyes are darting back and forth between you and their phone. Another way to tell is if the person is holding their phone away from you or if they are trying to hide it. If you are suspicious that someone is texting someone else, you can ask them directly.