Can one claim that a person is having cognitive distortion and be always correct?
The use of assumptions in clinical settings seems to play into the errors you mention. Cueball is just as prone to distortions as Megan.

According to Dawes, Faust, &amp; Meehl (1989), a professional in the field has to rely on one of two choices when making a decision on a client, either their own clinical judgment which is based on experience and knowledge or actuarial judgment which essentially rests on the statistical method, establishing a link between empirical data and the event in question. The clinical judgment does not seem to be accurate, for the most part. Clinicians fair better when using methods that are backed by actuarial methods, rather than relying on their own "hunches" or "gut feelings". 

It would probably be a better idea for Cueball to at least try to jot down some explicit instances of distortions and compare them to some baseline to ensure it is something out of the ordinary, at least for what he might know about Megan. 
Though he would be correct, at least in this instance, but it wasn't necessarily due to experience, but chance.
 Which is one of the big issues pointed to in the article by Dawes and his colleagues, they essentially find that the criticisms towards the therapist being an all-knowing being are valid and point out that they often forget to consider other variables that might be contributing to the situation, so with the occasional correct guess, they may begin to rely on their gut or worldview which distorts their decisions, overall. 

To combat these errors Lambert and Shimokawa (2011), highly recommended that client feedback is collected because it helps in understanding the client's subjective point of view and can aid in aligning the therapist's views to better meet the needs of their client, leading to stronger outcomes. The practice of feedback helps build a factor in therapy success, which is the therapeutic alliance (the relationship between client and therapist). Flexibility also helps as well.

This notion of client feedback is another layer on top of the common factors pointed out by Rosenzweig (2002) that seem to contribute most to positive client outcomes, more so than the theoretical method used. These factors include the qualities of the therapist and how they interact with the client and their specific qualities.   

Essentially, it is the nonspecific elements, like therapist and client qualities in interaction, that create the foundation for a better client outcome. 

Dawes, R. M., Faust, D., &amp; Meehl, P. E. (1989). Clinical versus actuarial judgment. Science, 243(4899), 1668-1674.

Lambert, M. J., &amp; Shimokawa, K. (2011). Collecting client feedback. Psychotherapy, 48(1), 72–79.

Rosenzweig, S. (2002). Some implicit common factors in diverse methods of psychotherapy. Journal of Psychotherapy Integration, 12(1), 5–9. doi:10.1037/1053-0479.12.1.5