I quit using Messages on my Mountain Lion Mac Mini long ago. It just seemed dysfunctional half of the time. It should check for new messages fairly often, as do my iPhone and iPad. But I would often not see an SMS appear for hours after the other devices had it. All three devices are on the same WiFi network, BTW. And it would fail to send SMS replies quite often. What's the purpose of having Messages on the desktop Mac if it doesn't work? It is quicker and more reliable to just use my iPhone.