There are a few different ways to do this depending on where you are within your account.
If your click on the "My Blog" link at the top of your account management portal, you will see a link on the right(next your blog name) that says "visit blog" - this link will take you to the live version of your site.
You may also visit your live site by hovering over your blog name at the top of your dashboard and clicking the link that reads "visit site".
If you want to see the live version of a specific post, there are 2 ways to do this.
From the Post Editor,
1. If the post is in "draft", click the "preview" button at the top right.
2. If the post has been published, click on the "view post" or "preview changes" link at the top of the editor.