Can the theory of evolution be tested? Evolution, when addressing common descent, is largely a historical science. This means that it relates to actions that are supposed to have happened in the distant past, and this makes testing the theory complicated because, unless time travel is invented, we cannot directly test the theory.
However, this does not mean that the theory is not testable at all. As with other historical investigations, you can make predictions and retrodictions (to utilize present information or ideas to infer or explain a past event or state of affairs - e.g., to "retrodict past eclipses" as opposed to predicting future eclipses) based on the theory.
What this means is that we can state that we would expect to find certain things (say, certain types of fossils) when looking at the historical record, and if those things are found, it supports the theory. Thus, while we cannot perform the kind of direct tests like we can in physics and chemistry, the general theory of evolution is testable just as other historical theories are testable.
http://atheism.about.com/library/FAQs/evo/blfaq_evo_science.htm

thanks to this website it explains how it's provable but over a long period of time