How do I know if my manuscript is actually ready?

Three checks that are more reliable than your own read-through. One: give it to five people who read your genre and don't love you. Not family. If three of them stall in the same place, that place is the problem — not the thing you've been fretting about. Two: read your first page out loud, then read the first page of a published book in your genre out loud. You'll hear the gap immediately if there is one. Three: can you say what the book is in one sentence, without the word 'and'? If not, it may not be finished structurally, however polished the prose is. Post your opening paragraph here if you want a read on it.