How would you know if someone use you as "login as" and try to alter data? I know that whenever you changed something in salesforce (e.g. Opportunity record) it will always records the login user as created or last modified id. So there will be no way to edit a record as someone else while you are login, unless you login as someone else.
Just to clarify your question are you asking if there is a way of checking if an administrator has used the 'login as' feature to sign into a user account and then modified a record?
If they have the 'last updated by' on the actual record that is updated will show the account that has been logged into by the admin. However you will be able to see the IP address of the login by user and the Setup Audit Trail will show admins logging in as other users.
Adminstration Setup>Security Controls>View Setup Audit Trail