From the position of the peaks in the XRD pattern. See for example:

http://rruff.info/Calcite/R050128

This .pdf may also be useful.

Hi, Saha,

Every crystalline materials have a characteristic XRD pattern. So, by matching XRD pattern of the materials with its standard XRD pattern you can identity that your sample is calcite or not. You can also perform Rietveld refinement of the XRD pattern using standard software.

dear Sudip Saha calcite phase can be identified from the XRD analysis— (PDF 00-047-1743) CaCO3 with thanks. as in the following sample;

please use any XRD matches programs , such as MDPI,match and crystal