A problem that arises in clinical practice is the difficulty to assess an isolated, deep layer muscle contraction through indirect methods and clinical skills with any degree of certainty.  Since the transversus abdominis and multifidus that you are trying to retrain are 1-3 inches deep, the only way to accurately test and measure progress is to use an imaging modality like diagnostic ultrasound. 

One study demonstrated that these subtle isometric contractions can be difficult to examine and assess, even for physical therapists who have had long experience, but minimal training related to this specific task.  Their research analysis showed poor agreement between palpation findings versus ultrasound imaging findings in this population of physical therapists. 

This study showed the appropriateness of ultrasound imaging as an extension of a physical therapist's clinical assessment of contraction in deep abdominal muscles.  Ultrasound imaging provides the ability to visualize the contraction of deep muscles in order to assess and problem solve the best feedback/facilitation strategy for correct activation.

If we want to objectively find the optimal facilitation/feedback technique for a particular individual, then we must assess and problem solve until we find it.  Ultrasound imaging can confirm that the facilitation/feedback technique chosen is valid.

What do the experts say?

My college football coach used to say:

When a patient leaves my office I know that the facilitation/feedback techniques that I have instructed them to use are appropriate for proper activation because they have been verified with ultrasound imaging.  Remember the quote: "Practice does not make perfect... Perfect practice makes perfect!"