Although economists have created very elaborate models to evaluate the consequences of changes in trade policy, here is a useful overview by Dani Rodrik of three conditions that lie behind any claims of large gains from trade:
(1) Is the trade distortion big?
(2) Will liberalization aggravate other market imperfections (such as those arising from technological spillovers) in the economy?
(3) Will liberalization worsen the distribution of income?
Try your hand at applying these principles to the case of agricultural trade barriers or visa restrictions before checking Professor Rodrik's assessment.