To check if one or more columns all exist, you can use set.issubset, as in:
if set(['A','C']).issubset(df.columns):
df['sum'] = df['A'] + df['C']
As @brianpck points out in a comment, set([]) can alternatively be constructed with curly braces,
if {'A', 'C'}.issubset(df.columns):
See this question for a discussion of the curly-braces syntax.
Or, you can use a list comprehension, as in:
if all([item in df.columns for item in ['A','C']]):