Venus is a 'twin' to Earth in terms of its size and its density.
Because Venus formed at the same time as the Earth, in the same general region of the solar system, and is very similar in both size (Venus = 6052km vs Earth = 6378 km) and density (Venus = 5.52 vs Earth = 5.24 g/cm3), it is believed that Venus has an internal structure quite similar to the Earth's.
Supporting data for internal models comes from gravity and magnetic field measurements from Venera, Pioneer Venus and Magellan spacecraft. It is believed that Venus formed a differentiated core with the heaviest elements such as iron sinking to the middle of the planet. It is not known if this core has yet solidified as much of the Earth's core has.
Although the proportions of core, mantle and crust may be similar to Earth, the surface evidence is that there are no moving 'plates' as there are on Earth which can mean that, either the crust is much thicker so that planet cannot form, or the mantle is not convecting as fast as Earth's mantle to move the plates around. Future spacecraft will deposit seismometers to search for 'earthquakes' that can help us probe the planet's interior.