Unfortunately, he diameters people usually look for are 27.2 mm (columbus SL or SLX. the good stuff), 26.8 mm (Columbus Aelle the medium stuff), or smaller the cheap stuff. so you cant tell much from mm level measurements, you need to get a precise measurement, to help you identify the level of frame tubing material.

EDIT: the circumference measurement is a pretty accurate method for measuring the diameter, as was indicated earlier. (any measurement error is divided by pi, so it helps to buffer the errors associated with imprecise tools like a tape measure).