really you should not be deleting records - but have a 'date left organisation' field or similar
you can have a simple message box with code along the lines of
Code:
if dcount("*","myTable","FKfield=" & me.PKField) >0 then
msgbox "This person cannot be deleted until all subordinates have been reassigned"
end if
other methods would be to disable the delete button so user cannot delete
Code:
me.btnDelete.enabled=dcount("*","myTable","FKfield=" & me.PKField) =0
Finally, as a backup set your relationships to enforce referential integrity - this will prevent a record being deleted accidentally