Indesign script that generates a text file with information on which pages each master page is applied to.

Check Master Page Numbers v.2.2.jsx

// Version 2.2
// https://gist.github.com/joonaspaakko/efc6eb11759965d54b20

// *************
//  Changelog:
// *************
// v.2.2.
//  - Changed some of the wording in the output.
//
// v.2.1.
//  - Tidied up the code just a tiny bit. Mostly just made it slightly more readable.
//  - Added colors to the text document to make it easier to read.
//  - Added a variable "joinRanges" at the top of the file that joins number rangers if set to true.
//    - This makes the output way cleaner.
//    - For example pages "1,2,3,4,5,10,11,12,13,14,26,30,31" will be output as "1-5, 10-14, 26, 30-31"
//    - FYI: Indesign supports number ranges like this in all inputs that take page ranges, like for example the "Apply master to pages" dialog.
//  - Added a variable "useDocName" at the top of the file. Adds document name to the text file if set to true.
//  - Text file is now saved in the folder containing the script file.
//  - Tested in Mac - Indesign CC 2018
//
// v.2.
//  - Now prints out the page information about every single page in the document.
//  - Removed the dialog, as it's useless now.
//  - Due to the increased number of text printed out at once, it is now printed into a rtf file.
//  - Tested in Mac - Indesign CC 2014
//
// v.1.
// - The first version of the script gives you the pages of a single master = https://gist.github.com/joonaspaakko/efc6eb11759965d54b20/02a53c865ec1f228b1646d4dbe558c5ccfa94040

var joinRanges = true;
var useDocName = false;

var masters_array = {};
var subMaster_array = {};
var docMasters = null;
	
if ( app.documents.length > 0 ) init();

function init() {
	

	var doc = app.activeDocument;
	
	// List of master pages
	docMasters = doc.masterSpreads.everyItem().name;
	
	// Creates same amount of arrays as there are masters ( [None] is also included ).
	// The arrays are populated later on.

	for (i = 0; i <= docMasters.length; i++) {
		masters_array[i] = [];
	}
	
	checkPages( doc );
	
	// Text file path
	// Points to the folder where the script sits...
	var rtf_file = new File( (new File($.fileName)).parent + "/Check Master Page Numbers"+ (useDocName ? ("( "+ doc.name +" )") : '') +".rtf" );
	
	print_Header( rtf_file );
	print_Body( rtf_file, doc );
	print_Footer( rtf_file );
	
} // init()

function checkPages( doc ) {
	
	// Loop through all pages...
	var docPages = doc.pages;
	var pagesLength = docPages.length;
	for ( var i = 0; i < pagesLength; i++ ) {

		// Current page
		var active_page = docPages[i];

		// Name of the master in current page
		// If master is not applied, use '[None]'
		var active_pageMaster = active_page.appliedMaster ? active_page.appliedMaster.name : '[None]';

		// Index of the master in current page
		// Index is 0 if it's '[None]'
		var active_pageMasterIndex = active_page.appliedMaster ? active_page.appliedMaster.index + 1 : 0;

		// Push page numbers to their respective arrays...
		masters_array[ active_pageMasterIndex ].push(' ' + (i + 1));

	};
	
}

function print_Header( rtf_file ) {
	
	rtf_file.open("w");
	// First line in the text file
	rtf_file.writeln('{\\rtf1\\ansi\\deff0\\line{\\colortbl;\\red0\\green0\\blue0;\\red255\\green0\\blue0;\\red153\\green153\\blue153;\\red31\\green165\\blue44;}\\i\\cf2 If you ran the script with this text file open, you may need to reopen it to see the changes.\\cf1 \\line\\line\\line');
	
}

function print_Body( rtf_file, doc ) {
	
	// Loop through the masters yet again
	// The point of this is to assing the correct heading to each line and their sub master info.
	for (i = -1; i < docMasters.length; i++) {

		var none = i === -1;

		// Get the current line heading
		var heading = none ? '[None]' : doc.masterSpreads[i].name;

		// Check if current master has sub master
		// '[None]' is never able to have sub masters, since it is not a master page.
		var appliedMaster = none ? null : doc.masterSpreads[i].appliedMaster;
		var appliedMasterIndex = !appliedMaster ? null : appliedMaster.index + 1;
		var appliedMasterName = !appliedMaster ? null : appliedMaster.name;
		
		var xerox = {};

		// Find out which master is applied to current master
		xerox.subMaster = appliedMaster ? ' ( Based on: \\b ' + appliedMasterName + ' \\b0 )' : '';
		
		var currentMasterPages = joinRanges ? getRanges( masters_array[i + 1] ) : masters_array[i + 1];
		var submasters = get_subMasters( doc );
		
		// Prints pages in the current master or if it has not been applied to any page, it prints out the text string.
		// In addition to the page numbers, it also prints the sub master of that specific master in parenthesis, if it has any.
		xerox.pages = currentMasterPages == '' ? '\\cf3\\i Has not been applied to any page' + (submasters[i] ? '\\i  directly, but it has been applied to other master(s)\\i0' : '') + '.\\i0\\cf1' : '\\cf4' + currentMasterPages + '\\cf1',
		// If current master is applied to any other master, this prints out the name those masters.
		xerox.parentMaster = (submasters[i] == null ? '' : '\\line\\b Applied to masters: \\b0 \\cf3' + submasters[i]) + '\\cf1';

		// Writes the text body...
		rtf_file.writeln( '\\b ' + heading + '\\b0 ' + xerox.subMaster + '\\b:\\b0  ' + xerox.pages + xerox.parentMaster + '\\line\\line' );

	}

}

function print_Footer( rtf_file ) {
	
	// Writes text footer...
	rtf_file.writeln('}');
	rtf_file.close();

	// Opens the text file
	rtf_file.execute();

}


function get_subMasters( doc ) {

	var submasters = {};

	// One master can be applied to multiple other masters....
	for ( index = 0; index < docMasters.length; index++ ) {

		// Sub master
		var appliedMaster = doc.masterSpreads[index].appliedMaster;
		// Sub master index
		var appliedMasterIndex = !appliedMaster ? null : appliedMaster.index;

		// If current master has a sub master and it doesn't have its own array yet, create it.
		if ( !submasters[appliedMasterIndex] ) submasters[appliedMasterIndex] = [];

		// Push current master name to sub master array.
		submasters[appliedMasterIndex].push(' ' + doc.masterSpreads[index].name);

	}

	return submasters;

}


function getRanges(array) {
  var ranges = [], rstart, rend;
  for (var i = 0; i < array.length; i++) {
    rstart = array[i];
    rend = rstart;
    while (array[i + 1] - array[i] == 1) {
      rend = array[i + 1].replace(' ','');
      i++;
    }
    ranges.push(rstart == rend ? rstart+'' : rstart + '-' + rend);
  }
  return ranges;
}