We perform testing of daily images before each milestone is released, in preparation for wider distribution. For information on testing the milestones themselves see ubuntu.com/testing.
Please refer to the Testing procedures for details, or if your new, go through the ISO Test Walkthrough. The alpha, beta and final milestones are tested according to a specific Release Schedule. Test reports are filed in the test tracker at iso.qa.ubuntu.com and bugs are filed in Launchpad.