The Earth is differentiated, as it has a distinguishable core, mantle and crust with different compositions and densities. Are all the other planets differentiated as well?
They are - somewhat by definition.
One criterion for a planet is that it is large enough that it is in hydrostatic equilibrium so that it takes on spherical shape. Any body large enough for that to occur will to a certain degree be a differentiated body as hydrostatic equilibrium means that masses need to be able to move, thus the heavier elements will sediment towards the center. Due to the speed of geological processes, differentiation (=de-mixing) may initially be faster, but is never really finished, unless a body is completly solid and cold (which no planet is).
The existence or non-existence of hydrostatic equilibrium is one of the differences between dwarf planets and so-called small solar system bodies.