Brian Steinberg Senior TV EditorAT&T and State Farm are the latest big advertisers to experiment with new ways of figuring out just who is watching TV – and the commercials that support it.The two marketers are among those who are taking part in an alliance between Discovery and Omnicom Media Group. Under the pact, a handful of Omnicom clients, including the telecom and insurance giants, will test new ways of tabulating linear TV audiences using Comscore and VideoAmp, and examine recent viewership data.