Thanks so much - good to know!
So when you are answering in a public poll, it will warn you before registering your answer that the answers will be public. If there is no warning then you will know that it is a private poll.
And as mentioned earlier, just look at the numbers - if they're not underlined, then it's private. So now everyone - feel free to answer the polls honestly - no other registered users will know what you've answered or even that you have answered at all.
ETA: This is how the polls work as far as I can tell. If I'm incorrect, please feel free to correct me.