Different plugin methods on different posts (by category)

I am developing a plugin using this structure:

if( !class_exists( 'My_Plugin' ) ) {
  class My_Plugin {
  // Methods
  }
}
if( class_exists( 'My_Plugin' ) ) {
  $my_objecct = new My_Plugin();
}
if ( isset($myobject) ) {
  // Filters & Hooks
}

I want is to add an extra methods (only on some cases) to the main class, like this:

if ( class_exists( 'My_Plugin' ) && !class_exists( 'My_Plugin_Extends' ) ) {
  class My_Plugin_Extends extends My_Plugin {
  // Other methods
  }
}

I am trying to verify on which context I am in order to use $my_objecct = new My_Plugin(); or $my_objecct = new My_Plugin_Extends(); but at the plugin loading moment it seems to be that I don’t have much information yet… ($post or $wp_query globals are empty). I would like to check if the post is on a given category (so I need the extra methods).

Suggestions…?