From version 4.0.35 all products in the products list will now have a supplier link on the right hand side menu.
Clicking on this supplier link will take you too a list of the supplier prices along with which is set as preferred.
If these need to be edited double click the required row and you will then be presented with a prices link which will allow you to edit and remove the preferred flag and also see when and how this was added by opening up the logs at the end of each row.
If you are not on version 4.0.35 or above please raise a case with the support team and we can get your system upgraded.