Use texture, grain size and mineral composition to separate igneous, sedimentary and metamorphic rocks.
Start with colour and overall texture, then ask whether grains are interlocking (igneous/metamorphic) or cemented clasts (sedimentary). Look for vesicles, fossils, foliation and reaction with dilute acid where appropriate and safe.
Never rely on a single trait. Combine hardness of constituent minerals, density feel, and geological context.