In order to preview what is actually printed, a WPF DocumentViewer can be used. Generally, this document viewer renders WPF FixedDocuments. Due to TX Text Control's special WYSIWYG rendering, TextControl can't be connected to this viewer directly.
Anyway, it is quite easy to utilize this out-of-the-box viewer effectively as a print preview control using TX Text Control's Page Rendering Engine. Each page is rendered on a FixedPage of the FixedDocument and displayed in the standard viewer.
Download the sample from GitHub and test it on your own.