How can you tell when a politician is lying?

Simple. Whenever they make ten year forecasts of anything, but especially tax revenues and government spending. Nobody knows what things will look like ten years from now. But they stick with the 10 year forecasts because ten years from now, nobody will remember how ridiculous their forecasts were. Make a one year forecast, and even the newsies will remember what it was, and dump on you if the forecast was wrong.