The following are some tell-tale signs that your device may be monitored. However, it's important to remember that these are indicators rather than definitive signs.
Slow performance - malware could be causing your phone to lag because it's sending your personal data, photos and documents to an external server
Overheating - your phone may be overheating because malware is constantly operating in the background
Abnormally high data usage - hackers may be using your data to remotely access your phone to transfer your information and files to an external server
Random reboots - a hacker may have remote access to your phone and is trying to reset the password and username for your online accounts so they can gain access
Strange text messages - a hacker may be using your phone's text system to communicate with their server or to encourage you to click on sinister links