  We present a method for the identification of heavily absorbed AGN (NH>10^23
cm^-2) from X-ray photometric data. We do this using a set of XMM-Newton
reference spectra of local galaxies for which we have accurate NH information,
as described in Brightman & Nandra. The technique uses two rest-frame hardness
ratios which are optimised for this task, which we designate HR1 (2-4/1-2 keV)
and HR2 (4-16/2-4 keV). The selection method exploits the fact that while
obscured AGN appear hard in HR2 due to absorption of the intrinsic source flux
below ~4 keV, they appear soft in HR1 due to excess emission originating from
scattered source light, thermal emission, or host galaxy emission. Such
emission is ubiquitous in low redshift samples. The technique offers a very
simple and straight forward way of estimating the fraction of obscured AGN in
samples with relatively low signal-to-noise ratio in the X-ray band. We apply
this technique to a moderate redshift (z~1) sample of AGN from the Chandra Deep
Field North, finding that 61% of this sample has NH> 10^23 cm^-2. A clear and
robust conclusion from our analysis, is that in deep surveys the vast majority
of sources do not show hardness ratios consistent with a simple absorbed
power-law. The ubiquity of complex spectra in turn shows that simple hardness
ratio analysis will not yield reliable obscuration estimates, justifying the
more complex colour-colour analysis described in this paper. While this method
does very well at separating sources with NH> 10^23 cm^-2 from sources with
lower NH, only X-ray spectroscopy can identify Compton thick sources, through
the detection of the Fe Ka line. This will be made possible with the high
throughput X-ray spectral capabilities of ATHENA.
