Since google and youtube are linked you can check your account settings and have it log your monthly activity. Also with gmail, if you scroll to the bottom, it'll show the ip addresses that have logged on to access your gmail. If one of those ips aren't yours, then someone has accessed your account. Yahoo mail also has last ip log in thing, but I believe that's in the top right corner. Facebook has an option that lets you log on from only safe ips/computers and you can turn that setting on so you're notified if an unauthorized computer tries to access your facebook account.