If, by some chance, you had a complete list of the characters, you could use a plugin that would render them in a way that made them stand out (e.g., wrapping them in span tags with a style the changed the font-size and/or color).
Even without the list, you could find them in various ways:
This looks promising (assuming that MODX is set for utf-8) as a plugin or snippet to use before export:
https://sourceforge.net/projects/phputf8/files/
require_once('libs/utf8/utf8.php');
require_once('libs/utf8/utils/bad.php');
require_once('libs/utf8/utils/validation.php');
require_once('libs/utf8_to_ascii/utf8_to_ascii.php');
if(!utf8_is_valid($str))
{
$str=utf8_bad_strip($str);
}
$str = utf8_to_ascii($str, '' );
Also ... This finds and strips non-printable characters, but could be modified to make them more visible:
$str = 'aAÂ';
$str = preg_replace('/[[:^print:]]/', '', $str); // should be aA
Something like this:
$pattern = '/([[:^print:]])/';
$replacement = '<span style="color:red; font-size:1.5 em">${1}</span>';
$str = preg_replace($pattern, $replacement, $str);
Another possible pattern for the above:
$pattern = '/([^\x20-\x7E])/','', $str);
