Like other types of computing devices, iPhones and other smart phones can sometimes be accessed by people who aren't authorized to use them. Someone could access data on your phone using malicious software, or someone could get physical access to the phone and access the data on it. Be on the lookout for unusual behavior on your phone, and use its built-in security measures and security software to keep intruders out.
Search for Signs of Hacking
One way of knowing how to tell if someone has hacked your iPhone is by looking to see if it's suddenly behaving unusually. If the phone is running slow, such as taking longer to start up, shut down or launch apps, that may be a sign of unauthorized software running on the phone to eavesdrop on you or monitor your activities. Similarly, you might notice the phone running hot to the touch or using more battery power than usual.
Alternatively, you might see the phone redirecting you to unwanted pages or app download sections of the App Store. In that case, you might have malware on the phone that's trying to make money by showing you ads or selling you unnecessary apps and services. This software could have been installed as part of an app downloaded from the App Store.
If someone who has physical access to your phone, such as someone you live or work with, has gained access to it by perhaps obtaining your login passcode, he may be looking through information on the phone when you leave it unattended. In that case, you may notice apps or files, such as photos, opening that you don't remember opening or searches showing in your history that you didn't make.
Keeping Your Phone Safe
Use the security features built into your iPhone to keep yourself safe. Set a passcode to control who has access to the device, and if you suspect that somebody's stolen your passcode, change it.
If you have an iPhone X, you can use Face ID to log in to your phone by facial recognition for added security. If you have an iPhone 6 or later model, including the iPhone 8 and iPhone 8 Plus, you can log in using your fingerprint with Apple's Touch ID feature. Either of these options will be difficult for someone to get around without your consent.
You can also install antivirus software on your iPhone to help keep malware off the device. Search the App Store for antivirus software that meets your needs.
If you suspect an app on your phone is compromised or behaving badly, consider removing it to see if the phone begins behaving normally again. You can also contact Apple or visit an Apple Store and ask "how to know if someone hacked my iPhone."