This appears like a question that is rarely addressed in field theory pedagogy (perhaps because the answer is obvious): how does one describe a particle of definite spin in quantum field theory?
For example, given some state in a theory of spinors (say a single particle for simplicity): $|\psi\rangle = \int \psi(p) a^\dagger(p)|p\rangle$, where $a$ is the ladder operator you obtain by canonically quantizing, say a Dirac field. Since this is a Dirac field, it describes a particle of spin 1/2. How does one extract information about the spin of this particle?. For example, what would the creation operator for a single particle of definite spin up look like?