There are several ways to verify unauthorized logins, but the strongest indicator that someone is logging into your account is that you have to log in repeatedly. Snapchat only allows one device to log in at a time. But, it should remember which device is already logged in so you don't need to visit the sign-in page upon opening the app. However, if you find that the app is prompting you to sign in, it may already be logged in elsewhere. If this happens often, you should immediately change your password and log out of all devices.