Everyone else has great answers, but I'm eager to add my thoughts.
How do you know if you're ready?
There are two major factors which I think tie into being ready for criticism.
- You think you're crafted of diamonds and your ink is molten gold.
I believe that if you think your writing is some of the best around when it hasn't been looked over by another, you're wrong. You need to be criticised and have other people look at your work because they might have different beliefs to you. One of my favourite parts about English Literature is hearing other people's interpretations. It's essential to hear other people's interpretations of your work rather than your own all the time.
- You can't see what's wrong.
Once again, it's the same as the last one. If you physically think your work is perfect but still haven't got it critiqued, you need it critiqued. It's all about hearing other people's interpretations.
- Before you do final edits and polishes.
This is what I do whenever I write anything. Even if it's a short story that I'm never going to publish, I edit it several times and give it to someone who can critique me. It's good practice. I think the best time to give your work to someone to critique, in my experience, is before the final edits. With what you learn from the critique, you are able to apply that in your work and improve it as much as possible in the final revisions of the manuscript.
How do you know if you're not ready?
- There are grammar issues.
This is a huge one for me. I have been asked to look over people's work before, and within the first 500 words there's at least one spelling mistake/grammar mistake. Comma splicing can be a very big issue if you do not identify it. When you actually do try and get published, if you have issues with grammar and spelling you're going to be put to the side. If you're trying to get critiqued, you don't want to critique to revolve around your grammar and spelling, rather the writing. It's so important to get rid of as many issues like these as you can to get the most out of your critique. Removing grammar and spelling issues also assists in getting your meaning across clearly.
This is quite self-explanatory and ties into the last one. If your work is unedited it will be of exceptionally low quality and unable to impress anyone. There will probably be ten thousand comma splices, five hundred spelling typos, six hundred little places where it loses flow. Also, if you're wondering why I comment on spliced commas, I have an issue with them so I have to pay immense amounts of attention to them during editing stages to make sure I haven't a single one in the finished product.
Getting critiques can be really exciting because you're able to hear other people's interpretations of your work. If you think your work is gold without any critiques, then you're wrong. It only appeals to you and not others.
I hope this helped.