How to check if Twitter bootstrap is loaded?

All you need to do is simply check if a Bootstrap-specific method is available. I'll use modal in this example (works for Bootstrap 2-4):

// Will be true if bootstrap is loaded, false otherwise
var bootstrap_enabled = (typeof $().modal == 'function');

It is not 100% reliable obviously since a modal function can be provided by a different plugin, but nonetheless it will do the job...

You can also check for Bootstrap 3-4 more specifically (works as of 3.1+):

// Will be true if Bootstrap 3-4 is loaded, false if Bootstrap 2 or no Bootstrap
var bootstrap_enabled = (typeof $().emulateTransitionEnd == 'function');

Note that all of these checks require that jQuery is already loaded.

See https://github.com/netdna/bootstrap-cdn/issues/111#issuecomment-14467221

<script type="text/javascript">
    // <![CDATA[
    (function($){

        function verifyStyle(selector) {//http://stackoverflow.com/a/983817/579646
            var rules;
            var haveRule = false;

            if (typeof document.styleSheets != "undefined") {   //is this supported
                var cssSheets = document.styleSheets;

                outerloop:
                for (var i = 0; i < cssSheets.length; i++) {

                     //using IE or FireFox/Standards Compliant
                    rules =  (typeof cssSheets[i].cssRules != "undefined") ? cssSheets[i].cssRules : cssSheets[i].rules;

                     for (var j = 0; j < rules.length; j++) {
                         if (rules[j].selectorText == selector) {
                                 haveRule = true;
                                break outerloop;
                         }
                    }//innerloop

                }//outer loop
            }//endif

            return haveRule;
        }//eof

        <!-- BOOTSTRAP JS WITH LOCAL FALLBACK-->
        if(typeof($.fn.modal) === 'undefined') {document.write('<script src="<?=$path_js?>/bootstrap.min.js"><\/script>')}

        <!-- BOOTSTRAP CDN FALLBACK CSS-->
        $(document).ready(function() {

            if( verifyStyle('form-inline') )
            {
                $("head").prepend("<link rel='stylesheet' href='<?=$path_css?>bootstrap.min.css' type='text/css' media='screen'>");
            }
            else
            {
                //alert('bootstrap already loaded');
            }
        });
    })(jQuery);
    // ]]>
    </script>

compatible with jQuery safe mode,

css check might need tweaks if you use custom css

if (typeof([?])=='undefined') { /*bootstrap is not loaded */}

where [?] would be any object or namespace which is defined inside the JS file itself.

The concept of "including" doesn't exist in javascript.

I would rather check for specific bootstrap plugin since modal or tooltip are very common, so

if(typeof($.fn.popover) != 'undefined'){
 // your stuff here
}

or

 if (typeof $.fn.popover == 'function') { 
   // your stuff here
  }

works in both bootstrap versions