I thoroughly agree with you. This "denial due to lack of undeniable evidence" has been one of my pet annoyances in discussions on this topic. I am all for skepticism, it is a fundamental pillar of science. What bugs me is the application of different standards when discussing the question "are humans sentient/intelligent" vs "are dolphins (elephants, apes, etc) sentient/intelligent". For both the cases we can only observe their external behavior, and I think all we can measure is whether that external behavior is consistent with our hypothesis. Sure, there are other hypothesis that explains the same behavior, the fault lies in choosing a less charitable hypothesis for animals but a more charitable one for humans.

Till better methods are discovered, I think a Turing test analogue is the best we can do.