DOCBOOK-APPS: Testing that xrefs point to xreflabels

Hi there

I'm wanting to check my docbook XML files to ensure that all <xref> tags 
contain valid links to <xreflabel> tags.

Before I dive into building something to give me a nice little list of 
un-resolvable <xrefs>, has anyone done something similar already?

Anyone got any tips?

TIA

Mart