UNITY — Ben Cottrell has been wondering what kind of apples grow on the trees outside his old farmhouse in Winterport, so Saturday he brought a handful to the Common Ground Country Fair. That’s where he found John Bunker, an expert in apple identification whose knowledge kept a steady stream of people stopping by his tent. Bunker, 66, of Palermo, took one look at the gnarly red and green samples Cottrell showed him and knew instantly. “They’re not in very good shape, but those are Northern Spy,” Bunker said. “Yes!” exclaimed Todd Little-Siebold, a history professor at College of the Atlantic who also studies apple identification. Little-Siebold said there are more than 1,500 types of apples in Maine.