How to know if an insance is a default instance or a named instance?

I obtain a list of instance names by using the method in this article:

http://blogs.msdn.com/b/askjay/archive/2011/10/11/how-can-i-get-a-list-of-installed-sql-server-instances.aspx

On my computer, there are two instance names under registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names\SQL

That is:

SQLEXPRESS   
MSSQLSERVER

Then how to know whether the above two instances are both named instance or one of them is default instance?

Thanks