You should have received a notice about the amount of your refund that was "offset" to pay student loan debt. If you don't have that notice, you can contact the Department of the Treasury's Bureau of the Fiscal Service (BFS) at 800-304-3107 to find out about the notice you were supposed to receive.
Another source would be the statement from the lending entity from which you obtained the student loan.
Lastly, you could deduce the amount of the offset by looking at your 2018 return refund amount compared to any refund you actually received.
**Say "Thanks" by clicking the thumb icon in a post**Mark the post that answers your question by clicking on "Mark as Best Answer"