Mrs/Miss Sabzevari,

Via single crystal X-ray diffraction.

If we compare the oscillator strengths of mostly ionic bonds (mostly inorganic materials) with those of mostly covalent bonds (organics), then the usual trend is that inorganic materials have much higher intensities and can thus easily be distinguished from covalent bonds. Also, as is well-known in infrared spectroscopy, molecular moities mostly keep their vibrational frequencies, often just the different local symmetries at the molecular site changes patterns of the free molecule.

But notice that the difference between mostly ionic and heteropolar bonds is not as clear as you might think. Ultimately, "ionic" and "polar" are pictures to visualize semiquantitative trends of quantum mechanical systems. There is not really a fixed divide...

Thomas explained the covalent and ionic bond very well with the FTIR, but if you study the UV-Vis-NIR spectroscopy for your samples then it will be very easy to specify the bonding nature for your samples by studying the nephelauxetic bonding parameter and Racah parameter (Alpha and Beta).

Thanks for all great information !