How to find roll-off from its transfer function equation? I want to find roll-off (unit dB/decade) of highpass filter $H(z) = 1 - a \cdot z^{-1}$.

I derived magnitude response to be

$1+a^2 - 2a\cdot \cos(\omega)$

To find out roll-off factor from above equation, how should I proceed?

I thought of doing 

$1+a^2 - 2a\cdot \cos(10\omega) \over {1+a^2 - 2a\cdot \cos(\omega)}$

But $\omega$ is a variable, how do I simplify this?
Your method will work, but you need to first find the cutoff frequency by setting the magnitude response to $\sqrt {2} /2$. Then plug that in for $\omega$.