How do I tell if a 2005 and 2009 Ford Focus are the same generation?

It will all depend on when in 2005 the Focus in question was built. Ford changed from the Mk1 Focus (LR model) to the all-new Mk2 Focus (LS model) in April of that year, so an early-build car will not be the same as the 2009 model, although a later-2005 one might be.

These two versions of the Focus were built on different wheelbases and platforms, so there may not be too much interchangeability. Certainly the bodies were quite different, so panels and interiors won't be interchangeable at all. Even the steering systems were different model-to-model. The Mk1 used conventional power-steering, while the Mk 2 switched to electro-hydraulic power-steering.

The compliance plate under the bonnet will tell you all you need to know about the car's build date. Visually, the earlier car had a much more angular style at the front with almost triangular headlights, while the later car was more upright and conventional looking.