The quarterly financial statements, and a knowledge of how Google accounting works[1]. Revenue from ads is correlated to CPC (cost per click), ad inventory is correlated to ads on Google sites vs ads on "other" sites, and margin pressure is correlated with operational expenses and traffic acquisition costs. The fuzziest number to model is the ad inventory and relies on estimates of search traffic, ads serviced, and reported revenue and CPC.

[1] I worked there and paid attention at their 'life of a dollar' class they used to give Nooglers. :-)