how to know which column is dropped by get dummies code example

Example 1: getting dummies and input them to pandas dataframe

note:
dummies = pd.get_dummies(df[['column_1']], drop_first=True)
df = pd.concat([df.drop(['column_1'],axis=1), dummies],axis=1)


note:for more that one coloum keep ading in the list 
dummies = pd.get_dummies(df[['column_1', 'column_2','column_3']], drop_first=True)
df = pd.concat([df.drop(['column_1', 'column_1'],axis=1), dummies],axis=1)

Example 2: get_dummies python

>>> pd.get_dummies(s)
   a  b  c
0  1  0  0
1  0  1  0
2  0  0  1
3  1  0  0