List the fonts used in all the Illustrator files in a folder

list_used_fonts.jsx

/*
List the fonts
This shows you all the fonts used in all the Illustrator files in a folder,
even if the fonts aren't installed on your system.
You can then replace them with this script: https://gist.github.com/2778305
*/

function getAllFiles(folder) {
  var fileList = [];
  function recurse (folder) {
    var files = folder.getFiles("*.ai");
    for (var i = 0; i < files.length; i++)
      fileList.push(files[i]);

    var folders = folder.getFiles();
    var len = folders.length;
    for (var i = 0; i < len; i++) {
      if (folders[i] instanceof Folder) {
        recurse(folders[i]);
      }
    }
  }
  if (folder != null)
    recurse(folder);
  return fileList;
}

// collect the fonts
var sourceFolder = Folder.myDocuments.selectDlg( 'Select the folder with Illustrator files in which you want to replace fonts');
var files = getAllFiles(sourceFolder);

var fonts = [];
for ( var i = 0; i < files.length; i++ ) {
  var doc = app.open(files[i]);

  var frames = doc.textFrames;
  for ( var j = 0; j < frames.length; j++ ) {
    var ranges = frames[j].textRanges;

    range_loop:
    for ( var k = 0; k < ranges.length; k++ ) {
      var range = ranges[k];
      var family = range.characterAttributes.textFont.family;
      var style = range.characterAttributes.textFont.style;
      var entry = { 'family': family, 'style': style };

      var elen = fonts.length;
      for (var e = 0; e < elen; e++) {
        var dentry = fonts[e];
        if (entry.family == dentry.family && entry.style == dentry.style)
          continue range_loop;
      }
      fonts.push(entry);
    }
  }

  doc.close();  
}

// Adapted from the Illustrator Scripting Reference CS5 page 216
// Creates a new A3 sized document and display a list of available fonts until the document is full.
var edgeSpacing = 10;
var columnSpacing = 230;
var docPreset = new DocumentPreset;
docPreset.width = 1191.0;
docPreset.height = 842.0
var doc = documents.addDocument(DocumentColorSpace.CMYK, docPreset);

var labelRef = doc.textFrames.add();
if (labelRef) {
  labelRef.textRange.characterAttributes.size = 10;
  labelRef.contents = "Fonts used in "+sourceFolder.fullName;
  labelRef.top = doc.height - edgeSpacing;
  labelRef.left = edgeSpacing;
}

var fontName = "";
var x = edgeSpacing;
var y = (doc.height - edgeSpacing - 20);
var count = fonts.length;
for (var i = 0; i < count; i++) {
  //sFontName = ;
  //sFontName += " / ";
  fontName = fonts[i].family + " / " + fonts[i].style;
  var textRef = doc.textFrames.add();
  textRef.textRange.characterAttributes.size = 10;
  textRef.contents = fontName;
  textRef.top = y;
  textRef.left = x;
  
  // check wether the text frame will go off the edge of the document
  if ((x + textRef.width)> doc.width){
    textRef.remove();
    iCount = i;
    break;
  } else{
    // display text frame
    if( (y-=(textRef.height)) <= 20 ) {
      y = (doc.height - edgeSpacing - 20);
      x += columnSpacing;
    }
  }
}
redraw();

You may have more success with this version of the script:

https://github.com/dyslexic-charactersheets/scripts/blob/master/List%20used%20fonts%20to%20CSV.jsx

It relies on the Tools.jsxinc and underscore.js files, so download those together.