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

Indesign - Master page numbers script

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

// Improvements?:
//   Currently the script checks every single page, even though it only prints results per one selected master.
//   Instead of that, it could print resuls for every single master on one go. This would be slightly easier
//   for the user and it probably wouldn't have an impact on the script performace.

// Current document
var doc = app.documents[0];

// List of master pages
var docMasters = doc.masterSpreads.everyItem().name;

// Dialog
dialog = "dialog {  \
    text: 'Master page numbers', \
    alignChildren:['left','top'], \
    decisions: Panel { \
        alignChildren:['left','center'], \
        group: Group { \
            dropdown: DropDownList { preferredSize:[200,24], properties: { items: ['" + docMasters.join("','") + "'] } } \
            check: Button { text: 'Check' }, \
            cancel: Button { text: 'Cancel' }, \
        } \
    } \
}";

var dlg = new Window( dialog );

var check;

// Check button click event...
dlg.decisions.group.check.onClick = function(){

   check = true;
   dlg.close();

   return check;

};

// Open dialog...
dlg.show();

// Check button was clicked....
if ( check ){

    // Selected master page
    var dlgMaster = dlg.decisions.group.dropdown.selection.toString();

    // This array is later on filled with page numbers belonging to selected master page
    var masters = [];

    var pagesLength = doc.pages.length;

    // Loop through all pages...
    for ( var i = 0; i < pagesLength; i++ ) {

        // Find out what master each page has
        var pageMaster = doc.pages[ i ].appliedMaster.name;

        // If page master is the same selected earlier in the dialog, push it to dlgMaster array.
        pageMaster == dlgMaster && masters.push( ' ' + ( i+1 ) );

    };

    // Print out the answer...
    alert( dlgMaster + '\n' + ( masters == '' ? 'Has not been applied to any page directly.' : 'Is applied to page(s): \n\n' + masters )   )

};