# How do know that Goldstone Boson actually become the longitude degrees of freedom in W+,W- and Z boson?

In many Quantum Field Theory text books they says these about Spontaneous Breaking and Higgs mechanism like this

In unitary gauge, the Goldstone Bosons are eaten by $$W^\pm$$ and Z and become their longitude degrees of freedom.

I don't understand how do we know that Goldstone Boson actually become their longitude degrees of freedom. How do we identify the longitude degrees of freedom for a boson mathematically? What kind of term can be viewed as longitude degrees of freedom? I am always confused by these unclear expressions in text books.