How can you tell if your phone has been hacked by RAT?

[Cupernicus]

Today while checking my emails on my phone, I noticed a message in the spam folder stating that someone had accessed my Riot Games accounts. It turned out to be true, as they had disabled the two-step verification on my account. About a month ago, I had downloaded an APK file, so I might have been infected with a Remote Access Trojan (RAT). How can I identify if I have a RAT and how can I resolve this issue?