It is possible to tell if someone's phone is not on by checking for certain signs. First, try calling the person's number. If you don't get an answer or hear a ringing sound, it could indicate that the person's phone is turned off or doesn't have service. Additionally, if the person usually communicates with you through text messages but suddenly stops sending them without explanation, this could also be a sign that their phone is not on. Finally, if you know where they are located and they are not responding to your calls or texts, it could be an indication that their phone isn't working properly or has been turned off.