An appearance test specification should depend on product and development knowledge, purpose of the test and attributes being evaluated, and regulatory requirements based on the product type.
Firms commonly employ standardized test methods such as those published in the compendia (USP, EP). For each type of visual evaluation, the method should define the test requirements including visual evaluation tubes, sample amount/volume, control articles to be used, and conditions for testing (viewing level, lighting, background).
Yes, the appearance test is subjective and QC analysts must utilize proper controls and be trained on how to properly perform the test.
For example, an appearance test for an aqueous solution may include separate evaluations of color, clarity, and may also include an evaluation for the presence of visible particulate matter (for parenteral drug product) so the specification may be “Colorless, clear, and essentially free of foreign particulate matter.”