Well, there's the obvious of running the spell/grammar check tool. It's imperfect, and more often than not I'm ignoring its suggestions, but it can help catch a few. One of the things that I'm doing right now is going through my manuscript and reading it out loud. I'm mostly doing this to check for flow, but it has also helped me catch a few mistakes. Also, a lot of word processors now have a speech tool, where it can read your writing out loud (usually achieved by highlighting a piece of writing and then right-clicking). I've found this at times to be helpful as well, because when people read, the mind automatically fills in missing words, so you don't even notice, but the speech tool is literally reading what is right on the page, so if you have it read your stuff out loud and then just listen, that'll help you catch mistakes like missing words and wrong words. It's very dry, though, so not the most fun way to go through your book.
The other thing I will say is that agents understand that mistakes do happen. This is why there are professional editors. They do expect your manuscript to be as polished as possible, but once in a grand while having a typo isn't going to make or break your manuscript. The issue is when there are constant mistakes littered throughout the manuscript, and the agent figures that your story just isn't worth the effort of cleaning all of that up. So obviously do your best to have everything in shape, but don't panic that one or two might slip through. I've had pieces I've been working on for years with maybe a dozen people who have read them and I still find mistakes when going through. Just do your best with it.