Determining whether an expression is less than or greater than zero.

Question

Let $x,n,$ and $\theta$ are greater than zero. How can I show whether the following quantity is greater than or less than zero?

$$\frac{n}{\theta^2}-\frac{\sum_{i=1}^{n}x_i^2}{\theta^3}$$

EDIT:

The quantity is actually a 2nd derivative of a function and I need to check that the stationary point is maximum. So, I have to show the quantity is less than $0$ everywhere.