If someone has lost interest in you, you will notice a shift in their behavior. For example, instead of texting you every day at the same time, they may stop initiating contact and invent excuses. They will suddenly become too busy or tired to text you. Instead of looking happy and enthusiastic to see you, they will seem bored - or even annoyed. Also, focus on their body language. For example, if their feet consistently point away from you, they have a closed body language - eg: their arms are crossed or they distance themselves from you - and they avoid eye contact while they used to make eye contact with you before, they probably don't like you anymore.