how to distinguish between the sandstone, siltstone and shale by their appearance?

texture and particle (grain) size. Sand is obviously not clay. The difficulty only lies with the marginal situations, where grain size is in that borderline between sand and silt, or silt and "clay" (using clay as a grain size classification rather than a mineralogical one).

Shale is made from "clays" (very fine sediments) and has a fine bedding as a consequence. Sand is made from coarse sediments and tends to have a blocky massive bedding. Silt is in between; still somewhat blocky but not so massive. these are gradational differences so there are gray areas.

Field designations are not the same as what a quantitative analysis might indicate. field identification is based on judgement of appearance; quantitative analysis is based on where the numbers lie on the scale.