You would not be notified if anyone logged into a new device with your account information. I would suggest that if you are concerned with someone accessing your account you can add a passcode to your account. This will be an extra layer of protection that is needed beyond your password (and will also be required to access the account in any AT&T retail store).
You can add a passcode by visiting a local AT&T Comany Owned retail store.
Hope this Helps!
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.