True. It cannot be "proved".
There is no "proof".
The misconception is that if it can't be proved, it isn't valid.
In science, theories are never proven to be true. Proof is something that happens in geometry, not science. The only proof of which science is capable is proof that a theory is false. That will happen when something is verifiably observed that undeniably contradicts an essential feature of the theory.
There is a mountain of evidence from multiple, diverse scientific disciplines ( to be presented below) that has not been shown to be false.
That's how theories are put to rest .
Although theres no one smoking gun that proves evolution, ONE smoking gun can disprove it.
Hasn't happened in over 150 years!
And, by the way, scientific theories ARE tested and discredited all the time. See http://en.wikipedia.org/wiki/Superseded_scientific_theories for examples .