Missing words are difficult for the author and sometimes even editors to catch, but not an alert reader. When reading over your manuscript, do so slowly enough to note each word is indeed written as opposed to assumed. Reading it aloud may help, but not necessarily.

I suspect that most authors think much faster than they type, making it easy to skip over words. When you’re on a creative roll this is especially true, when you can hardly get the thoughts down fast enough, before you lose them. Nonetheless, like invisible typos, missing words will throw readers out of the story, something you want to avoid. In some cases, a good grammar checker may catch them, but test it to make sure. If you use beta readers, make sure they keep an eye out for such things, too.