Nothing to add to the excellent elimination advice, but i would add that recognising the early on-set is a good step to saving the aircraft.  One dead givaway that often occurs is a very audible buzzing caused by the rapid high frequency surface oscillations - whether aileron or whole wing. If you hear it decelerate immediately. More violet twisting will actually be visible if you look carefully on a flyby - it will show up as shimmering of the colour of the covering - you might not notice it unless looking for it, Any change in expected control response or unexpected control input not given by you, can be another clue. Sadly, flutter is frequently so quick and violent that by the time you relise whats going on, it too late.  I had a scratch built 9 foot thermal soared years ago that got terminal flutter, i was gently diving her to get better penetration when i heard the buzz, a second later i could see the wing twisting around the right tip, another second and the wing wasoff! The cause was very light building and insufficient torsional rigidity.
It also showed up recently on one of those ultralight foamy shockflyers i had - very wide ailerons of very twisty foam - a bit of excess speed and the buzz set in followed by a rapid partial roll - faster than the normal roll though, more of a jerk really. However, a lightweight foamy worth pennies is nothing to get excited over, but on a big 3D money pot would make me sweat some.........