Arguably, the easiest way to show entanglement is to plot a wavefunction against two variables. If the plot is a product of them, the state is a product state. If not - it is entangled.

As writing a wavefunction as a matrix $|\psi\rangle_{ij}$ is the the crucial step in Schmidt decomposition, we call such plots Schmidt plots.

Let us consider two states:

entangled: singlet state $|\psi^-\rangle = (|01\rangle - |10\rangle)/\sqrt{2}$,

product $(|01\rangle - |00\rangle)/\sqrt{2}$.

They may look seamingly similar, but the later can be decomposed into a product $|0\rangle(|1\rangle - |0\rangle)/\sqrt{2}$.