"All species are evolving and transitional."
How do you know?
Because we see evidence for this ongoing process everywhere we look. Correlating evidence in numerous fields; biology, paleontology, bio-chemistry, genetics, molecular biology, zoology, botany. We see it occurring right before our eyes in ring species. Every living thing can be fitted into a nested hierarchy of characteristics. The fossils that we can find match the predictions that we make from the hypothesis that the nested hierarchy indicates that all of life is a tree from a single original ancestor long long ago. The molecular evidence, including errors and non-functional DNA matches the same predictions. The predictive power of the theory of evolution is enormous. God could have "designed" things to be that way, true, but only if God wanted to fool the observant into thinking that the process that He used was evolution.
Anyone who thinks that all of life on earth is descended from what could be put on a wooden boat a few thousand years ago needs much faster (several orders of magnitude faster) speciation to have taken place since then than anything that evos suggest is even vaguely possible in order to create the 20 million+ species that we can see now.
Evolution is not proved in the strict geometric or number-theory sense. Nothing in the real world ever is because we have no axioms or pure inductions in the real world. We can't even prove objectively that the universe was not created last thursday, intact with all of our memories (like Adam's navel). But we can be as sure that evolution is true as we are of any objective fact that can be checked with observations of the world around us. No scientific knowledge is more soundly based on the data available.