Can someone post theory to decide how many real roots the cubic equation possess

Mainly deciding between whether the equation have one real root or 3 real root

Every polynomial function on differentiating, reduces one real root. So differentiate the cubic equation, you will get a quadratic. Decide the no. of roots from discriminant. If it has two real roots, then eventually the cubic will have three, if the quadratic has none real roots then cubic will have only one real root.