re 233 post on repairs
Most common technique with composites is ultrasound. Simplified version of factory techinque where an xyz gantry shoots multiple streams of low pressure water jets on large piece surface, using ultrasound transducer and computer enhancement of ' echo" returns to determine thickness, debonds, internal flaws.
Such as a drafting pencil dropped on surface by accident as a prepreg tape layup machine was laying down plies. Not discovered until after autoclave and trimming of final part ( major wing upper surface as I remember- which was quite thick ) . it was left in place . . .
In some cases- infrared tomagraphy(sp?) can also be used to find void areas when used with hot air guns.