Use the name property to get the name or the label of a Series.
>>> df = DataFrame("admissions_train")
>>> gpa = df.select(["gpa"]).squeeze()
>>> gpa
0 4.00
1 2.33
2 3.46
3 3.83
4 4.00
5 2.65
6 3.57
7 3.44
8 3.85
9 3.95
Name: gpa, dtype: float64
>>> gpa.name 'gpa'