The chlorophyll in green vegetation reflects shit loads of near infra-red radiation but hardly any red so surfaces with a high infra-red to red reflectance ratio are likely to contain green vegetation. its called NDVI (http://en.wikipedia.org/wiki/Normalized_Difference_Vegetatio...) and its a very common way of detecting vegetation on earth from space. Don't know how well it'd work when you're just looking at a tiny planet billions of miles away tho...
