The only foolproof-ish way of knowing what is inside a tablet is tests done by some rather precise, expensive equipment that separates elements into their component parts (chromatography), and then analyzes the mass of the separated components (spectrometry) and compares them to a database of known components. Nuclear magnetic resonance (NMR) spectroscopy is also an option.
It is unlikely most people will have access to equipment with the precision necessary at home. There is an organization out there (https://www.ecstasydata.org/) that does anonymous GCMS tablet tests of MDMA in particular for $40 (and other substances for a higher price).
The typical tests out there for most people are reagent tests -- chemicals known to react with a desired substance to turn a certain color. Reagent tests are not foolproof -- the main thing you know is that if you do not get the desired color, you most likely don't have the correct product. It is possible to fool a reagent test. They are better than nothing for sure.
Taste, color, and crystallization are not really reliable ways of identifying the product.