You're not quite ready for an editor if:
It’s your first draft. You deserve all kinds of accolades for finishing. Yay, you! But don’t call an editor yet. Instead, put your manuscript away for at least two weeks.
Then, bring it out again and read through it with fresh eyes. Fix all the things that need fixing — and believe me, these things will jump out at you.
Read it through once for plot, and then for character, and then for structure, and then for setting. Go through it again to correct any errors of grammar, spelling, or punctuation. Yes, that means you need to read through it five times, at least. If you want someone else to do this work, then hire a ghostwriter.
You’re too close to the material. At some point, your manuscript becomes a commodity as well as your baby. Your editor wants your work to be the best it can possibly be, and that often means revising with a machete as well as with a scalpel.
That can be hard to take, especially if you’ve written a memoir. If the thought of making changes makes you defensive or sad or angry, then you’re not ready to work with an editor.
You don’t know what your book adds to the world. If you can’t answer the “why” questions (why you wrote this book and why anyone should read it), then answering the “how” questions (how all its different parts answer these questions) is much harder.
Your editor can’t read your mind, and if your destination is uncertain, then your editor will not know how to make sure you end up in the right place. Know what you want to accomplish through your work before you hire an editor.
As Stephen King says in his brilliant On Writing: A Memoir of the Craft, "Only God gets it right the first time, and only a slob says, 'Oh well, let it go, that's what copy editors are for.'"