To tell if your phone has malware you can use malware security scan apps like Avast/AVG Mobile security, Norton 360, Malwarebytes Mobile Security and even Bitfender Mobile Security, which offer features like real scanning, web protection, anti-theft, and even VPNs, with options for both paid versions and free versions to help detect and remove threats on android and iOS devices.
Some phones also may have build in security scans like Google Play Protect.
Also you can look out for signs of malware on your phone which include overheating, sudden poor performance and slowness as well as rapid battery drain, aggressive pop-up ads, unexplained data usage, unexpected app installations, apples crashing, strange messages or calls from your phone number.
Common signs of phone malware are apps suddenly and constantly freezing or crashing, phone slowing down or the phone overheats.
Contacts receive spam or strange messages from your number.
Apps you didn't install appear or your homepage and search engine changes.
Excessive, unwanted pop-ups or new tabs while browsing appear.
Spikes in data usage and unexplained increase in mobile data usage.
And the battery on your phone depletes much faster than it usually does, as malware runs in the background.
To remove malware from your phone, first reboot your phone in Safe Mode (Android) or check app settings (iPhone), then uninstall suspicious apps, run an antivirus scan, clear cache/data, disable browser notifications, and if all else fails, perform a factory reset as a last resort, ensuring you back up important data first.