How can you tell if someone else has power over your phone?

You can tell if someone else has power over your phone if you notice unusual activity, such as unexpected messages being sent, apps opening or closing on their own, or changes in settings that you didn't make. Additionally, if your battery drains unusually fast or you find unfamiliar apps installed, these could be signs of unauthorized access. Checking for remote access apps or monitoring your data usage can also help identify if someone else is controlling your device.