  In this paper we are concerned with hypoelliptic diffusion operators
$\mathcal{H}$. Our main aim is to show, with an axiomatic approach, that a
Wiener-type test of $\mathcal{H}$-regularity of boundary points can be derived
starting from the following basic assumptions: Gaussian bounds of the
fundamental solution of $\mathcal{H}$ with respect to a distance satisfying
doubling condition and segment property. As a main step towards this result, we
establish some estimates at the boundary of the continuity modulus for the
generalized Perron-Wiener solution to the relevant Dirichlet problem. The
estimates involve Wiener-type series, with the capacities modeled on the
Gaussian bounds. We finally prove boundary H\"older estimates of the solution
under a suitable exterior cone-condition.
