In some cases, running an UPDATE statement in production can save the day. However a borked update can be worse than the initial problem.
Short of using a test database, what are options to tell what an update statement will do before running it?
In addition to using a transaction as Imad has said (which should be mandatory anyway) you can also do a sanity check which rows are affected by running a select using the same WHERE clause as the UPDATE.
So if you UPDATE is
UPDATE foo SET bar = 42 WHERE col1 = 1 AND col2 = 'foobar';
The following will show you which rows will be updated:
SELECT * FROM foo WHERE col1 = 1 AND col2 = 'foobar';