Researchers create new nanocomposite everyday and doing experiments on it and presenting their findings. How to identify the applications of newly created nanocomposites or what factors determines the application of new nanocomposite.
The question is very general in nature. I wish you could have given a context or example.
But broadly speaking, the application will depend upon the type and properties of the precursors i,e, the matrix and 'nano' filler. The physical, mechanical, chemical or electrical properties that are enhanced by formation of nano composites should be studied well by characterization. While developing a nano composite the researcher will anyway have a targeted property that is to be enhanced.