How do I validate a tinyMCE editor, if it is blank by appending a string next to it?

getContent() should work just fine. Your fiddle doesn't contain the form validation code for the editor value, which is quite crucial here. Try this:

var editorContent = tinyMCE.get('tinyeditor').getContent();
if (editorContent == '')
{
    // Editor empty
}
else
{
    // Editor contains a value
}

Forked fiddle

Also note you've declared multiple id's for your select drop-down.

Edit: You can get the id of the editor container with the getContainer() method: tinyMCE.get('tinyeditor').getContainer(). Inserting an error message after the editor would then be something like this:

$('<span class="error">Editor empty</span>').insertAfter($(tinyMCE.get('tinyeditor').getContainer()));

This, however, will create a new span each time the user clicks the submit button, so you'll probably want to have an error message container with a unique id and check if the container already exists before inserting it.

Edit 2: Updated fiddle.

You can do this to check if the content is empty without parsing html:

var content = tinymce.get('tinymceEditor').getContent({format: 'text'});
if($.trim(content) == '')
{
   // editor is empty ...
}

What you want can be easily done. Her is a link to a fiddle with my solution.