I actually see this question quite a lot - someone writing their short story/novella/manuscript is just about finished, and they start thinking about the next step. It's natural to get that feeling to call an editor once those last words are typed and finalized, but that might be a little hasty. I offer this little checklist to go over before getting an editor, and some of the steps should start before the work is even finished.
Drafting. Once you finish your work, you have completed the first draft. Pro tip: No editor wants to do a hard edit on a first draft. This should be the phase where you get a few people to give it a read to see if they like the story, if they enjoy the characters, etc., but you don't call an editor. More to the point, some people suggest stepping away from the work for a few days or weeks to provide some intellectual distance. Let the preconceived ideas of what you have written wash away, then reread it with a fresh, critical eye. This usually reveals all too quickly why editors hate first drafts, and should show you a bunch of things that need reshaping.
Rewrites. A 70,000-word novel might seen like a big accomplishment, but only the author should know about the 100,000 words that it started off with, the 40,000 words that got changed, the 20,000 words that were added, then the 50,000 words that were ultimately deleted. This is the rewrite process, and you never want to bring in an editor before you have gone through this step. Why have an editor correct sentences and paragraphs you will delete anyway? No - your first few run-throughs are yours alone, changing scenes, characters, and plot arcs all in the name of hammering out a pretty sound story.
Cut out the fat. I don't remember the author, but some famous writer said that the difference between a good book and a great book is that the great one is shorter. The general meaning of this is that a writer should dedicate one read-through to cutting out unnecessary words, sentences, descriptions, and even sections. This may sound brutal, and it is, but it forces the writer to really consider what is important to the story and what was just a fun scene to write. It might be difficult to eliminate beautiful but meaningless descriptions, but it creates a greater focus on the storyline and a more intense experience for the reader.
Are we ready to hire an editor now? Not quite yet. We have one more step to take - what do we want from the editor? An editor will gladly do a spellcheck and a grammar screening while fixing all our semicolons, but is this all we want? This is our opportunity to have an outside person tell us if our drafting and rewrites paid off. All the questions that came up in the workshops - were they addressed? Did we create the intensity we had hoped for with our brutal cuts? We need to decide if that editor is going to provide us with any feedback, or just polish the work we are already confident in. We need to have a very firm idea of what we need from those outside eyes.
That's when we get an editor.