I did a search (on Direct TV) and saw that the first two preseason games have appeared on the TV schedule. For the purposes of this thread, I will just post the national showings (games that appear on the NFL network)
Raiders @ Vikings
8/8/14 - 10:00 pm to 1:00 am
8/14/14 - 10:00 pm to 1:00 am
Cardinals @ Vikings
8/17/14 - 12:00 am to 3:00 am
the times that I listed are in the Mountain Time Zone so adjust accordingly.
I also haven't had much luck in finding out schedule that conveniently lists all the preseason games....