And it is. Very easily. It is tested all the time. Every new genome sequenced, every fossil found, every body (or part thereof) that gets its anatomy analysed,... Every single one of them is a test for the model.
And the model keeps changing.
When I think of evolution theory and all the changes that must be made as new discoveries are made, I'm reminded of Ptolomy.
Something else I think about when people mention evolution theory is my old 2001 Chrysler 300m. Imagine some race of beings coming to our planet a couple of million years from now, long after mankind is extinct or has left the planet a Mars-like hulk. They find my car perfectly preserved in ice or many layers of rock, etc.
Now, imagine they find a 2001 Dodge Neon from the same year, but catastrophies have placed it in a layer of rock they determine to be a million years older. As they look at the makeup of both cars, they notice that they share the same, exact door handle (they do). When analyzing the qualities of the cars and the suggested age of each, they deduce that the 300m evolved from the Neon. And to further complicate things, they later find a 2001 dodge intrepid and announce they have found the missing link!
Except what really happened was this:
1. Both cars were designed for human occupants.
2. Both cars were designed to be effectively useful for humans to transport humans and some of their stuff.
3. Both were designed for the same road conditions and environment.
4. Both were designed by the same company with the same general goals.
5. The company believed in efficiency of production and economy, and saw no reason to design two separate door handles when one design and manufacturing process was sufficient.
i.e. the similarites between species do not prove evolution. They prove design.
"They're trying to find themselves an audience. Their deductions need applause." --- Peter Gabriel (The Lamb Lies Down on Broadway - Genesis)