How do you know when a manuscript is finished? Is it a feeling? That perfect sentence that just lingers there with your pen hovering over it, knowing that if you mess with it the whole thing can come crashing down?
Every writer reaches that point when they don't think they can do any more on their work, but if it's good enough for them does that mean it's good enough for the world or their agent?
If you have the confidence to let it go and let others judge your work then it might be time to do so. Some writers are perfectionists who have great difficultly letting their manuscript go to someone who might give criticism because they are afraid of the response. And sometimes they are afraid that what might be their best work won't be good enough.
All of these insecurities are felt by writers everywhere. It's something we all share in common.
Whether it is agonizing over a punctuation mark or a rhyme scheme, or a final piece of dialogue, there comes a time when we have to decide it's ready.
The sooner you tie up those loose ends and let it go, the sooner you can receive feedback and move onto the next part of the process of writing.
Ultimately you have to trust yourself, trust in your talent and you'll make the right decision.