I would like to use the jQuery UI style Sheet, but I don't know whether it's pre-installed in WordPress or not.
Without knowing that I think it could create a problem. If it is available in WordPress, in that case
wp_register_style() will become problematic because first we have to
wp_deregister_style( $handle ) and then register our script, otherwise the two stylesheets may conflict.
Even if I'm asking a wrong question just give me some idea or resources where I can find available scripts and styles, so that I can easily find out what are the styles and script I need to register and enqueue in my plugin.
One more thing I don't have to do useless code like registering a stylesheet if it is already available.