The temperatures at which pure substances melt are well known, and are published values. Our Celcius temperature scale is based on the melting point of ice which occurs at 273 degrees above absolute zero. We call this 0°C.
It is common practice to confirm the identity of a substance, for example the product of a reaction, by measuring its melting point and comparing it to the published data.
It is also possible to tell how pure the substance is by how ‘sharp’ its melting point it. A pure substance melts at a single temperature, but an impure substance (a mixture) melts over a range of temperatures, and the wider the range of temperatures between where it starts to melt and when it is finished melting, the more impure the substance is.