what is the way to know what was Thales statement ?

Answer 1

He had two theorems: The first had to do with a triangle inscribed in a circle, with one of the triangle sides being the diameter. The second, was his statements on proportional line segments:

http://en.wikipedia.org/wiki/Intercept_theorem

Answer 2

If you really want to explore how his name got attached to these statements:
http://journals.tc-library.org/index.php/hist_math_ed/article/view/189/184

An excellent article. Yes, we have no evidence that Thales actually discovered these principles.