How to find out whether a PhD dissertation is ready to submit?
You should talk about this with your advisor and your consultant professors. You can complete your report and show it to your advisers. They will read and tell you what lacks in your report. Also, you may talk to the members of your PhD committee orally before giving them your report. They may give you some guides or some of their expectations.

Talk to the previous PhD students and see what brought them to the point that their PhD was ready to be presented.

Another important thing is your PhD proposal in which you have clarified your goals of your dissertation. If you have completed your promises in the proposal (or you have strong reasons why could not reach some of your goals), then your work is completed.

Another thing may be regulations of your department. Some departments require their students to submit one or two journal or conference papers out of their PhD project. You can check your department's regulations on what are their expectations from a complete PhD project.