  Let $f(x)=x^8+ax^4+b \in \mathbb{Q}[x]$ be an irreducible polynomial where
$b$ is a square. We give a method that completely describes the factorization
patterns of a linear resolvent of $f(x)$ using simple arithmetic conditions on
$a$ and $b$. As a result, we determine the exact six possible Galois groups of
$f(x)$ and completely classify all of them. As an application, we characterize
the Galois groups of irreducible polynomials $x^8+ax^4+1 \in \mathbb{Q}[x]$. We
also use similar methods to obtain analogous results for the Galois groups of
irreducible polynomials $x^8+ax^6+bx^4+ax^2+1 \in \mathbb{Q}[x]$.
