Much like the folk putting echos into find and wildcarded shell commands to check the output, I'll often start manual sql updates by doing 'SELECT something FROM database WHERE...' and check the output rows match my expectations before hitting the up arrow to replace the SELECT with an UPDATE.

For bigger tables, I use COUNT(something) if I expect it to be long but I have an idea of rows affected, or LIMIT if that's going to give me an idea that it's doing the right thing.