No, babies are very different from adults in this regard, inasmuch as adults can tell us that they are in pain, can describe the pain, etc.
This doesn’t look like a big difference to me. Moreover, adults may also be unable to speak due to various illnesses or disabilities.
For very dissimilar entities, like rocks, the probability of being conscious would fall back to some kind of prior, though I don’t know how such a prior could be justified. (Purely intuitively it seems clear that rocks being conscious is highly unlikely, but it isn’t obvious why.)
… really? You can’t think of any reasons for this belief? Just pure intuition, that’s all you’ve got to go on? Are you seriously making this claim?
Yes. At least not from the top of my head. Note that this prior is supposed to not incorporate the information that you are conscious yourself.
Congratulations, you just reinvented philosophy. :)