I feel a distinction needs to be made here. Proof is not necessary to believe in something. What is required is evidence that can withstand scrutiny. So as far as other people's sentience goes, there is evidence around me every day that indicates that they are indeed sentient.
Even if the open windows of science at first make us shiver after the cozy indoor warmth of traditional humanizing myths, in the end the fresh air brings vigor, and the great spaces have a splendor of their own - Bertrand Russell
