You learn about calls or texts you didn't make

“You might also notice calls and texts that you haven’t sent to numbers in your list of contacts. Ensure that you monitor this activity closely as some of these could be premium-rate numbers that malware is forcing your phone to contact with all the proceeds landing in the pocket of the person who has compromised your phone.” –Rob Webber, mobile expert, and CEO and founder of MoneySavingPro.com
You get spammy pop-ups

“Another sign of a compromised smartphone is spammy pop-ups or weird screensavers. While not all pop-ups indicate that your phone has been infiltrated, an increasingly high number of pop-ups could be a sign that your phone has been infected with a form of malware called adware that forces devices to view specific sites that drive revenue through clicks.” –Rob Webber, mobile expert and CEO and founder of MoneySavingPro.com
You are getting security messages

“If your phone has been hacked you might notice some unusual activity, such as security messages notifying you that your email or social media account has been accessed using a new device, password reset links, or verification emails saying that you have signed up to new accounts that you are unfamiliar with.” –Rob Webber, mobile expert and CEO and founder of MoneySavingPro.com