Hum just to correct myself. For a province to be counted as nomadic province, the capital slot does not have to be empty, since for instance nomadic capital occupy this slot (and if you don't have any other holding in the province, it is counted as nomadic land). I don't know if it is the case when the capital slot is occupied by a tribal, castle, city or bishopric, but I can't see why it should be different.
So I guess, the only criterion for determining that a province is nomadic, is that it has no more than 1 slot occupied by a holding (and has to be in a nomadic realm of course).