Engine teardown or more flight hours after the damage has been incurred (and engine failures) may be needed to know.

NASA writeup of what the damage looks like even though it may be undetectable initially:

http://www.alpa.org/portals/alpa/volcanicash/03_NASADC8AshDamage.pdf

Just because no one has died yet from commercial airliners ingesting ash doesn't mean they could re-use the airplane.

How would overhauling the engines after each flight raise costs???