How to detect Debian systems w/o root passwd ?

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear Debian boot,

It is now possible to set up Debian system without a root password.
On such system sudo must be used instead of su.
What is the canonical way to detect such situation ?

My purpose is to change su-to-root to automatically default to sudo mode
on such system.

Cheers,
-- 
Bill. <ballombe@debian.org>

Imagine a large red swirl here.