How to to test the concentration of melatonin in a strong acid? If a small tablet of melatonin was coated with a substance to ensure a slow dissolution in water and was then placed in a strong acid, neglecting the limits imposed by maximum saturation, it would slowly but surely dissolve. I would imagine the simplest method for determining exactly how much has dissolved would be by testing its concentration, however the information available on the topic uses methods beyond my capacity to preform as a high school student. Is there any relatively simple method to determine the concentration?
Is there any relatively simple method to determine the concentration?


That depends on your equipment. Having the following at hand would be helpful:


a UV/VIS spectrometer
mortar and pestle
UV-grade ethanol (or methanol)
pure melatonin


How to proceed:


Prepare a stock solution of the pure melatonin in ethanol.
Prepare some dilutions for the expected concentration range.
Measure the UV absorption of all solutions at $\lambda = \pu{277 nm}$.
Draw your calibration curve (absorption vs concentration).
Powderize the tablet using a mortar an a pestle.
Dissolve the powder in a defined volume of ethanol. 
Measure the UV absorption of the sample.
Determine the concentration in the sample using your calibration curve.


You have just applied the Lambert-Beer law. Others have done the same and reported the method in Pharmacophore 2014, 5, 252-257 (free PDF).