The test balls were sent out during the end of the 2014 season. I was one of the lucky recipients of a sleeve. From what I understand is that it is fairly random and the only thing that can increase your chances is being a active user of these forums / message boards and Team Titleist will contact you and send you the test balls. It is something that is so great and unheard of from such a large prestigious company as Titleist, they already have the #1 ball in golf but strive to keep themselves on top and keep themselves as close to their customers as possible. Team Titleist

As Robert said.  Since Titleist is on a 2 year cycle, they will be identifying some large number of folks to test the NXT through DT balls in 2015 for the early 2016 release.  If you have Pro-V in your profile, likely you will be waiting until the cycle begins for the 2017 release.  Stay active and you may get an e-mail letting you know plain white sleeves with balls labeled Test are on the way.

As others have said, there's no real criteria to being selected but being an active member of the forum helps. I was lucky enough to be able to test the new Pro V1's last year and it's an amazing ball. Can't wait to get out and play with them.