Those lovely punkbirders have put together some helpful tips on identifying cormorants.
I have to say that the method is a little tough to use in the field. I have scratched lines on the objective lens of my Kowa and that seems to do the trick (but don't do this at home...).
Also, the method is far easier than the Lars Svenson formula to identify Iberian Chiffchaff
'Svensson’s ‘multiple character value’ for Iberian Chiffchaff is designed ‘with ringers and field workers in mind’. To get the value you must first add seven measurements: wing length; bill length – to the skull; distance from p1 (outermost primary) to p2; distance from wing-tip to p6; distance from wing-tip to p7; distance from wing-tip to p10; distance from wing-tip to s1 (outermost secondary). Then you must substract two: tail length and the distnace from p1 to the primary coverts. He writes this as MCV = W + B + (p1<p2) + (p6<WT) + (p7<WT) + (p10<WT) +(s1<WT) – T – (p1>p.c.). For males, a MCV higher than 73.2 should mean Iberian; an overlap area between 71.9-73.2 exists; anything below 71.9 should equal Common; for females the discrimant value is 70.9.'
I am looking forward to the Collin's Mathematical Guide to Bird Identification, whereby its doesn't really matter what a bird looks like...