The ABA Techshow Blog had a great GMail tip a couple of weeks ago. The post explains how you can tell if someone is accessing your GMail account from another location:

Once you’re signed in, at the bottom of the page you’ll find a lot of fine print.  Beneath where it indicates how much of your allotted storage space you’re currently using, you’ll see a line that gives the time and IP address of your last account activity.  Click on the “Details” link, and you’ll be taken to a window showing the last several times anyone signed in to your account, how (Browser, mobile, POP3, etc.), the IP address, and the time.  It will also tell you whether there is concurrent activity.  There’s also a button to allow you to close any concurrent sessions and link to allow you to change your password immediately.

I know that most people have a GMail account that they use for some purpose. Doesn’t it make sense to check this every once in a while to make sure someone else isn’t accessing your account?