Probably one of the hardest parts of assessing for a patient's directional preference with repeated motions is knowing if a patient is actually responding to the motion positively. It's not that it's difficult for us, as therapists, to know, it's that patients often struggle to properly communicate what they are experiencing.
There are several things to look out for when assessing repeated motions. One, is a decrease in pain. Obviously if the pain feels better, we want to continue with the motions. Second, is centralization of symptoms. This one can be somewhat difficult, because a patient may experience symptoms in an area they previously had none. The pain could get worse proximally, but if it improves distally, it is a positive sign. Both an improvement in motion and motion before pain occurs can be positive as well. We are loading the directional preference with repeated motions, which often has a loss of motion to start, so an improvement in mobility is highly desirable. All of these things, if seen, can be positive with either the repeated motion or an asterisk sign (the motion or activity that the patient has pain with). What you want to watch out for is when symptoms stay peripheralized or worse for 5 or more minutes when done.
Why this can be difficult, is that we can only observe so much as therapists for these findings. Some of the markers are subjective. You'll likely notice that patients with signs of central sensitization or a fixation on their MRI findings are so convinced that what you are doing won't help (or will make them worse) that they don't even give the assessment/treatment a chance. This is why it is essential to first thoroughly explain how beneficial the treatment can be and what exactly they should be looking for. Some education of the lack of correlation between pain and imaging findings likely is beneficial as well. Finally, don't be afraid to continue with up to 100 or more repetitions when assessing. Likely, 20 repetitions alone may prove insignificant, but over time the 100 may make a significant change and lead to a more confident result. Be sure to educate your patients on what exactly they should be looking for and don't be too quick to give up!