MovableType Blog : See the URL you use to login to the blog and simply replace mt.cgi with mt-xmlrpc.cgi (i.e. http://www.myblog.com/cgi-bin/mt/mt-xmlrpc.cgi)

WordPress Blog : Take the base URL of your blog (i.e. http://www.myblog.com) and append /xmlrpc.php at the end (i.e. http://www.myblog.com/xmlrpc.php)

BlogSpot (Blogger) : Take the base URL of your blog (i.e. http://example.blogspot.com) and append /feeds/posts/default at the end (i.e. http://myblog.blogspot.com/feeds/posts/default)