Except when a particle is spin-0, field of all particles transforms when frame of reference is changed, and this defines what spin is. The question is, specifically how does the quantum field transform in the corresponding equation, and how does this relate to the quantity of spin? I want to see how this works mathematically (specifically, I want to know spin-1/2 and spin-1, obviously.)
A good exposition of fields with arbitrary spin and their transformation behavior is in Weinberg's Vol. 1 on QFT. The special cases of spin 0, 1/2 and 1 are treated there, too.