How can you know if you are ready ?

I have been investing a lot of my time into the GMAT preparation. And
I am not sure whether I am ready or not. English is not my first language. RC is one of my weaknesses, especially those scientific long text with a lot of details. I can do sometimes very well on CR and sometimes just do as badly as I never practiced them before. I realized that it depends on my mental concentration and how do I feel on the specific day. My first try to the GMAT was in August 2004 with a 500 (Q44, V16) and with a lot of preparation. I worked on my weaknesses since then, just did the OG Verbal section again , all the questions step by step, analysed all my mistakes, i can even look to the questions and know the answers right away. I did PR CAT exams, got a 620-600-630 and 630, Kaplan CAT exams got a 580-580-540 and 590. I do not know if i am ready, I have been asked to provide a 600 plus score ...