Three helpful ways to know if your phone has been infected with viruses:

Check the app manager. If you find an app that you did not install before, remove it immediately. It may be a malicious app that is stealing your data. If necessary, you may need to reset your device.
Check the apps that are consuming a lot of battery power. Go to Settings >> Battery and see if there are any strange apps that are consuming a lot of battery power. This is a sign that your device may be infected with viruses.
Check the apps that are running when connected to the internet. If you find any apps running automatically that you did not install, turn off the internet and remove them.