I don’t think you can – I know my perception of the world is not the same as everyone else’s perception; but how this links to consciousness is a question that I don’t have an answer to. Luna or Melanie may have a much better idea!
You can’t. There is absolutely no way of knowing for sure. However, we have some indications that it might be reasonably similar: The structure of your sensory organs and of your brain is really very similar to everybody else’s, so the way you sense and process information is really quite similar. Also, you can talk to other people, you can often guess how they are feeling and you can relate to their thoughts and experiences. This doesn’t prove that you are experiencing consciousness in exactly the same way, but there is at least some common ground, I think.