detect device used

Hi everyone! I need to add to my WP site a different css code, related to if it viewed with a pc or a smartphone.
I need to write something like that:

     .post
       {
	    if(user is using smartphone)
            {
                width: 100%;
            }
            else
            {
                width: 150%;
            }
       }

how can I do that???
thank you in advance! Bye!