I live in Flanders and don't really go riding green lanes in the south so I can't really help you with tips.
But in Belgium forests are generally off limits to motorized vehicles. In the south there can be some exceptions on private land, but you would need permission of the owners to ride there.
Sometimes there are large organized rides/rallies like the Trans-Ardenaise for which special permissions are granted through regions that are normally forbidden.
If you stick to the farmland trails and gravel roads that are not marked as private of have other prohibitive signs you should be ok.