The theory of evolution is falsifiable
Evolution suggests that when faced with selective pressure, the frequency of alleles in the population will change. Over time, pressure for the maintenance of these frequencies or simply divergence due to isolation will lead to speciation. Both these claims can be challenged, but both can be demonstrated to occur. The example of the peppered moth demonstrates that selection pressure alters allele frequencies. The transposable P element in drosophila demonstrates that speciation can occur, and has done in our lifetimes.
Demonstrating that either of these was uncommon would allow evolution to be discredited. This wouldn't be difficult, but it hasn't happened.
The theory of evolution is predictive
Evolution makes one powerful prediction - over time, organisms will become more suited to their environment. It does not make any predictions about the manner in which this will occur, hence the wide range of different solutions to the same problem.