Simple way is to ask directly. A reputable dealer will tell you. A small drop of hydrochloride acid applied will react with malachite but that can't be used because you do not own the stone and that might mar the stone. Some unscrupulous sellers represent it as malachite but give it a different name like Detroit Malachite.that is made up but it is one way they try to mislead you.
Genuine malachite is heavy and cold to the touch. Other substances (they make imitations with resin and powders from real malachite) such as glass and faux malachite will warm quickly in your hands. Just be careful. Ask the dealer if it is genuine malachite. Lastly type in your browser, is my malachite genuine. Several sites talk about how to tell the difference.