I actually was looking at this recently. There used to be some foolproof ways to identify ABP, but they removed them - as a bug fix IIRC. They definitely don't want to be identifiably.

But there are a few heuristic methods, basically just loading an image from a blocked domain (it's a sure thing double click will be blocked), waiting, then checking the DOM to see if the element has been removed. Not fool proof, or specific to AdBlock but good enough.

I have a couple of detection demos (including flash block) here:

http://dsingleton.co.uk/code/firefox-plugin-test-suite/