Quality is primarily determined by external appearance, juice content, and acidity levels. Buyers should look for lemons with firm, thin skins and a bright yellow color, as these typically yield more juice. In international trade, lemons are often graded as Class I (high quality, virtually blemish-free) or Class II (slight skin defects allowed). Ensure the juice content is at least 25-30% of the total weight and check for uniform sizing (e.g., counts of 75, 95, 115 per carton) to meet retail or food service standards.