iOS: Sign Me Out is a helpful iOS app that'll let you remotely check up on your Gmail and Facebook accounts to see if you're signed in from any other locations. If you are, you can click a big red sign me out button to disconnect a potentially unauthorised user.
Any account you add to Sign Me Out will give you a count of any activity you might want to be aware of. Tapping an account will bring up more information and allow you to sign out anyone currently logged in to that account by simply pushing a button.
If you buy the $1.19 full version of the app, you can add is many accounts as you want. If you just want to give it a try or only need to use one account, the free lite version will work just fine for you. Both are available right now in the iTunes App Store.