A good way to test the efficiency of organic sunscreens would be to monitor the absorption of light by the material. A good way to do this would be to coat transparent substrates with the same amount of different types of sunscreen. These substrates can then be placed into a spectrophotometer, which can measure the amount of light transmitted through the sunscreen. An ideal sunscreen would transmit all visible light (so you don't look purple or green, or whatever),but would not allow any UV light to transmit through. To conduct a thorough analysis, one could also characterize the chemical content of the sunscreen after the transmittance testing to see if any dangerous compounds are generated during the test. This can be done using mass-spectrometry or infra-red spectroscopy. Another test might also evaluate the ability of the sunscreen to stay effective while you are swimming. In this case, the transparent substrate covered in sunscreen could be submerged underwater to see how easily it can be washed off.