In addition to being a writer, I have worked as a copy editor for fifteen years. I worked exclusively for one publisher, and they had a Style Manual that I consulted. For things that weren't in the style manual, they wanted me to use the Chicago Manual of Style.
When I first started working for them, I would receive hard copies of the text to copy edit. I had to mark changes in red and attach sticky notes to direct queries to the editor, design people, or author.
Once we moved into the digital age, I received text on disk or as an email attachment. At this point, it became much easier to catch the most common errors because I could do a Search for the wrong thing and replace it. For example, this particular publisher wanted to use contractions to impart a sense of informality. Instead of "you will" they wanted "you'll." Instead of "that is" they wanted "that's" and so forth. So I would search for the un-contracted forms and change them.
This is something we can all do as writers. There are words that we tend to over-use or misspell or misuse, and a search will point these out to us. Past perfect construction (using "had") is undesirable, so do a search for "had" and evaluate each case to make sure you really need to use it.
Do you have trouble with "its" versus "it's" or "there" versus "their"? Do a search for each of those and evaluate whether you have used them correctly (have the "rules" right next to you so you can consult them if you need to).
Search for the letters "ly" to see if you are using too many adverbs. (Not all adverbs end in "ly" of course, but it's a start.)
If you want to make the best possible impression on editors or agents when you submit your work, it pays to fine tune some of the "little things."
I'd love to hear what the rest of you writers search for (or realize you *should* search for) in your manuscripts!