For computing, at least computationally, the Julia set of the mapping $z_n = z_{n-1}^2 + c$, one only needs to follow that $|z_n|\leq 2$ for all $n$, as in the wiki page.
Now, if I have a quadratic rational function
$$ f(z) = \frac{a_0 + a_1z + a_2z^2}{b_0 + b_1z + b_2z^2}, $$
what would be the equivalent condition to find the Julia set?
How this condition is derived?