They fly it to see whether it flutters, and if it does, they redesign it.
That's quite a frivolous way of putting it, but this is from this week's
Flight Internationa regarding the 777-300ER which is starting its flight test programme:-
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">The first flight kicks-off a 12-month effort expected to cover around 1,600h of flight tests and around 1,000h of ground tests for both aircraft combined. Most of the hours will be amassed by WD501 which will immediately be used to clear the initial flutter envelope. Boeing hopes to obtain type inspection authorisation by the end of February, by which time brake tuning and systems tests will be well under way. By March, WD501 is due to begin almost two months of aerodynamic performance test work, most of which will be based at Edwards AFB. (emphasis added)</font>