On Fri, 24 Aug 2012 17:47:51 +0000 (UTC),
(Andrew Gabriel) wrote:

It's also to check that both sides of the double check valve
are working

How would you test this? Remove the tap and apply water through the
"wrong" end with a hose pipe and if it comes out the drain hole, the
first valve is broken; if it comes out the "back" of the tap, both are
broken?

I've never heard of testing them before. Is it done on installation or
is it supposed to be done annually?