Integrated Security Question

Is there a variable that the username is stored in upon connection to the database. What I want to do is have anyone on this network be able to get into this application without typing in their login information again (which I have already done). After that, I would like to check to see if they are authorized to use it by comparing their user ID to a table of User IDs in the database. If there is a variable this is stored in please let me know.