Understand the structure
Check the number of rows and columns, the meaning of each field and whether the data covers the expected period.
Inspect missing and duplicated data
Measure missingness before filling values. A duplicate row may be an error or a valid repeated event; understand the grain first.
Python
import pandas as pd
df = pd.read_csv('transactions.csv')
print(df.shape)
print(df.dtypes)
print(df.isna().sum())
print(df.duplicated().sum())Check plausible values
Inspect minimums, maximums and unexpected categories. Document decisions so another analyst can repeat your preparation.