Check for Twitter Bootstrap Loaded

There is no good way of doing this.

One way is to enumerate the already registered scripts, like this:

    function yourplugin_add_my_stylesheet() {
      global $wp_styles;
      if ( $wp_styles instanceof WP_Styles ) {
        // enumerate all current styles
        foreach( $wp_styles->queue as $handle ) {  
          // use the $handle to see if it's called 'bootstrap'
          $obj = $wp_styles->registered[$handle];
          // or use $obj['src'] to see if `bootstrap.css` is in there
        }
      }
    }
    add_action('wp_enqueue_scripts', 'yourplugin_add_my_stylesheet');

But this will in most cases not work:

1. The order of plugin loading is not know, so the offending other plugin my load after this.
2. The offending other plugin my not use terms like ‘bootstrap’, or may load the css via other means, not the wp_register_style() method.

A possible suggestion is that your plugin gets a option page with a checkbox that allows the admin to either load the css or not. This way, if there is a conflict or duplication then one can switch off the loading of the duplicate.