If you have an AR(p) process like this:
$y_t = c + \alpha_1 y_{t - 1} + \cdots + \alpha_p y_{t - p}$
hen you can build an equation like this:
$z^p - \alpha_1 z^{p - 1} - \cdots - \alpha_{p - 1} z - \alpha_p = 0$
Find the roots of this equation, and if all of them are less than 1 in absolute value, then the process is stationary.