In chemistry, there are several ways of identifying unknown substances in the context of receiving product and grading it. We can be sure that customers are receiving the prescribed product by testing batches as a quality control measure.

A trained chemist can determine unknown substances by performing tests on samples. If a sample is soluble or not, its melting point and so on. Equipment can test sample and determine substance.
Some are...

* Spectrophotometer - uses light absoptionabsorption and reflection to determine a substance.
* Raman spectroscopy - uses raman scattering to determine substance.
* Mass spectrometry - uses the mass of atoms in a magnetic field to identify a substance.
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Different types of spectroscopy and spectrophotometry are well known and widely used to identify and quantify compounds in the field of research as well as in the industrial and chemical laboratories. For example, in chemistry, and pharmacy, UV-visible spectrophotometry is a basic technique to analyze samples based on the application of the Beer-Lambert-Bouguer Law. In biochemistry and molecular biology, spectrophotometric analysis is essential for determining biomolecule concentration of a solution and is employed ubiquitously for determining the concentration of DNA, RNA, or protein. In clinical laboratorieslaboratories, both manual and automated spectrophotometric are extensively used for the determination of blood, urine, and body fluid samples.

Source: https://www.intechopen.com/chapters/79874

Other

* Gas Chromatography (GC)
* Gas Chromatography Mass Spectroscopy (GC-MS)
* Ion Chromatography (IC)
* High Performance Liquid Chromatography (HPLC)
* Dispersive Raman Spectroscopy
* Fourier Transfer Infrared Spectroscopy (FTIR)
* Crystallography: X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD)
* Thermogravimetric Analysis / Differential Scanning Calorimetry