Custom field Rules implementaion
Simple UI solution:

Add a condition of "Data Comparison"
Data selector - commerce-line-item:type
Value equals - "Modification" (or whatever your custom line item type is called)

Now when you add a condition that checks for a field, all the fields on that custom line item type should appear. Example, "Data value is empty - commerce-line-item:custom-field".
Thanks to trevorkjorlien comment at Drupal.org