It is difficult to determine whether a wine is natural or not, since every winery has its own approach. Some prioritise a holistic approach; others focus on a lifestyle or on a work philosophy.
In any case, calling a wine or sparkling wine natural implies a very low level or no added sulphites – sulphurous acid. You will find fruit and only fruit in the wine. The main work is carried out in the vineyard, not in the cellar.