Even when your data uniquely defines which layer the top or bottom layer is, the layer orientation cannot be determined without an inscription in the copper, since nearly every CAD program permits a mirrored output of the data (this is even true for single-sided circuit boards).
Note: This is not a special case just for Multi-CB, it applies worldwide for every printed circuit board manufacturer.
This is the TOP layer of a circuit board. Which (layer) orientation is correct (right or left)?