It depends upon the geometry and surface characteristics of the product in question. Unfortunately you cannot have the same type of visual inspection for all types of products. Once you identify the product you are interested in then it will be possible to identify the methods. See a few surveys for typical products here:
Newman, T. S., & Jain, A. K. (1995). A survey of automated visual inspection. Computer vision and image understanding, 61(2), 231-262.
Hassan, M. H., & Diab, S. L. (2010). Visual inspection of products with geometrical quality characteristics of known tolerances. Ain Shams Engineering Journal, 1(1), 79-84.
Baudet, N., Maire, J. L., & Pillet, M. (2013). The visual inspection of product surfaces. Food Quality and Preference, 27(2), 153-160.
Ravikumar, S., Ramachandran, K. I., & Sugumaran, V. (2011). Machine learning approach for automated visual inspection of machine components.Expert Systems with Applications, 38(4), 3260-3266.