Parallel lines and optical illusions

Look at the 2 lines above, are they parallel or bent?

Look at the diagonal lines above, do they look parallel?
Optical illusions occur in everyday life. Sometimes things do not look as what they are supposed to be. It is important to illustrate this point to students learning geometry, that one should not assume some properties of the object by visual inspection.

In short, for questions not drawn to scale, one should not make unnecessary assumptions. So if you're given two lines, how can one determine whether they are parallel?