From a recent Masters thesis at my place of employment:

"Canadian Forces (CF) CF188 Hornet fighter aircraft have been plagued
with water ingress and related damages in the composite flight control
surfaces. Different Non-Destructive Testing (NDT) inspections such as
neutron radiology (which include neutron radiography and neutron
radioscopy), X-radiography, IR thermography and Ultrasonic testing, have
indicated various types and stages of failure in the graphite/epoxy skin
and aluminum honeycomb core. Water ingress is considered to be the
origin of damage such as corrosion, delamination and disbond, and is
found predominately in rudders; hence, water ingress detection in
rudders is of foremost importance and is the focus of this study.

Although studies have shown that, in principle, neutron radiography is
considered to be the superior NDT method for detecting water ingress in
flight control surfaces, the effectiveness of the system has never been
quantified. A quantitative measure of system effectiveness or system
reliability provides a methodical approach to evaluating system
capability. Moreover, system reliability information can be used to
find a Safe Inspection Interval (SII) to inhibit the onset of corrosion
and other defects due to water accumulation.

A methodical and intensive study of four techniques was carried out in
accordance with the Nondestructive Evaluation System Reliability
Assessment Handbook (MIL-HDBK-1823) to generate Probability of Detection
(POD) curves to assess the reliabilities of the NDT systems for
detecting water inside rudders. The study concluded that, using the
neutron radiology system at RMC, neutron radiography was determined to
be the most reliable NDT method employed by the CF and that neutron
radiography could detect a water volume as low as 6.85 ?L for 90% of the
time with 95% confidence. Reliability of neutron radiography for water
detection in the rudder was quantified at 6.85 ?L, compared to 8.27 ?L
for neutron radioscopy, 7.33 ?L for IR thermography, and 28.2 ?L for
X-radiography. Furthermore, this study has revealed a common trend for
water entry and migration."

The only catch is that you need access to a nuclear reactor (we have a
SlowPoke) to do this.

Cheers
Marty