I think we can very strongly infere that other humans are conscious from knowing that we are conscious ourselves and seeing that other peoples brains work in the same way as ours and people think and behave in a very similar way as us and also have a concept of what consciousness is that corresponds with our own. In my opinion believing that other people aren't conscious is in the same category of beliefs as the past, the future or even the world and physical laws not existing outside of our subjective experiences. Sure you can not mathematically prove any of that but questioning it isn't really useful for more than a philosophical exercise of scepticism. As for AI I think it is a tricky question that I can't answer. Right now I don't think it is but maybe in the future it will be.