The best way to tell if someone is interested in a casual relationship or something more serious is to observe their behavior and communication style. If they seem focused on fun, lighthearted activities and don't express any interest in getting to know you beyond the surface level, then it's likely that they are looking for something more casual. On the other hand, if they take time out of their schedule to get to know you better and make an effort to establish a deeper connection with you, then chances are they are interested in something more serious.