How can ? we mesure Planck constant h ? or is it a definition?

Answer 1

E=hv
E -energy of an electromagnetic wave
v - frequency of that wave
h - Planks constant

Since h=E/v and both E and v are measurable than h is also measurable...

Answer 2

Planck found an approximate value by fitting the black-body curve to his theoretical description. It can also be found by looking at the slope of the energy vs frequency in the photo-electric effect. Since it comes up in so many different ways in subatomic physics, it can be measured with almost any experiment in that area of study.