• As Jamie wrote. Have a specific, curated set of data. Know the opening state, make the change, query for the end state, which you should know.

    To do this right, you should have multiple cases, NULLS, blanks, negatives, etc. Test how your code handles a variety of beginning cases, with rules for how things should end up.

    Note that you won't catch everything at once. Write a few cases you know, but add tests as you discover new issues.