Discussion of all views on demarcation of science would be beyond scope of Economics.SE so I will focus on the mainstream Popperian view of demarcation of science (but as mentioned Economics would clear virtually all established views for more info on that have look at Ross Philosophy of Economics, or Hausman Philosophy of Economics: An Anthology).
"labor supply elasticity with respect to income tax is -0.2" is science/scientific statement because the statement could be disproven by evidence which would show that labor supply elasticity with respect to income tax is not -0.2.
"if you want to reduce CO2 emissions by $x$ metric tons there ought to be $y\%$ carbon tax" - is a scientific statement because we could find by empirical observation that $y%$ carbon tax does not yield reduction of emissions by $x$ metric tons.
"Take from the rich and give to the poor" - is not a science. No amount of empirical observation can disprove statement: "Take from the rich and give to the poor". You can make infinite number of experiments, have god like omniscience and be aware of all facts, but nothing can show that statement is false. That statements is unfalsiable. It is expression of value/ideology/political view but not science.
"Take from none, and allow the Invisible Hand to decide the winners and losers," is again not science. No amount of empirical observation can falsify that statement. Even if there would be crystal clear evidence that laisseslaissez faire improves all aspects of life or that it leads to poverty or ruin cannot disprove that statement.
"we should increase taxes by 10%" - is not science as nothing again can refute that statement. No matter of empirical observation on disadvantages of taxation or on other hand benefit of public spending funded by taxation can ever disprove that statement.
"people should not smoke" - again expression of value not scientific statement
"smoking causes cancer" - this would be in the domain of science (although not economics in this case but I wanted to throw one more down to earth example).