Let the Werner state $\rho_W = W\mid\Phi^+\rangle\langle\Phi^+\mid + \frac{1-W}{4}\mathbb{I},\ W\in [0,1]$. I have repeatedly heard that such a state is separable if $W\leq\frac{1}{3}$ and entangled otherwise. I know the definition of entanglement, but am having trouble working with the density matrix form. Could someone show me where to begin?