With this script for Adobe InDesign you can determine the x-height of a text passage.
To measure the x-height, mark the relevant text passage in the layout of your InDesign document and click on the »Measure« button. The determined value is displayed in the script panel.
For control purposes, the title bar shows you the font family that is used, the style and the size in points/millimeter.
Of course, such a measurement is only meaningful if the text passage is represented by the corresponding font and not by a substitute font. A small yellow triangle indicates when a font is not available on the system. A warning also appears if the text frame and the font are scaled.