I once had to implement Arabic support in an existing editor. It also had to support mixed RTL and LTR input. Interesting to see how the cursors walks differently through the text when going going forward and backwards and how text selection works. Most document editors (MS Word, Libre Office with CTL, for example) and browsers implement this correcly as well. So, you can just compare if a text is displayed correctly by comparing how it looks in a document editor or a text box in an HTML file.