Hard to generalise - depends on degree of MR overspeed. However, you should look closely at the head and take a torch up there with you looking for any signs of stress near the bearings. If it feels well out of balance at start-up and there were no previous reports, then have it checked by an engineer or at least an experienced pilot/instructor.
Rippled blades very rare.
Engine overspeed is easy to spot - the cooling fan nut. If the white lines don't line-up, then it has likely been oversped at start-up.
"Faliure mode" - again depends on amount of overspeed. You would most likely (with MR) encounter severe vibrations well before failure.
Engine - loss of power. Do a power check at 100% on the ground. If you don't see 13" MAP (on a Raven II), then talk to an engineer. Oversped engines suffer from valves hitting pistons, bent stems and therefore loss of compression. (Plus damaged bearings at the crank/con rod whic is less obvious).