Click 'Admin' on the top right of the screen.
On the left hand side, under 'Database Management', click 'Field Management'
On the right hand side, enter your the field label of the field you're searching for (1)
Select the field from the search results. Any references will appear on the left hand side, within the field details (2).
From there you can determine the appropriate next steps (ie. confirm a field is good to delete in SFDC, update Marketo references based on a SFDC change, hide a field in Marketo, etc. )