Determining whether motion under a central force is stable

The question asks me to use the effective pontential to show that the motion in a circle of radius $a$ under a central force $\frac{b}{r^{2}}+\frac{c}{r^{4}}$ per unit mass (with b and c positive) is stable if $a^{2}b < c$. How would I go about doing this? What's the condition required for an orbit to be stable?