We derive a simple sufficient-statistics test for whether a nonlinear tax-transfer system is second-best Pareto efficient. If it is not, then it is beyond the top of the Laffer curve and there exists a tax cut that is self-financing. The test depends on the income distribution, extensive and intensive labor supply elasticities, and income effect parameters. A tax-transfer system is likely to be inefficient if marginal tax rates are quickly falling in income. We apply this test to the German tax-transfer system, and we find that the structure of effective marginal tax rates is likely to be inefficient in the region where transfers are phased out.