Answer:
No one except the Google Database knows the real Page rank of a webpage. It changes every day dynamically. All we get is a final PageRank update in a cycle of approximately three months. This is called the Pseudo PR. If it displays 3 for a particular web page then it could be actually 3.24 or 2.67 or some similar value. Again no one has any idea except Google as to how the PR is rounded for the final update to toolbars and public PR database.
If its the pseudo PR you are interested in the best way is to install a Google Tool Bar... alternatively you can get Mozilla Firefox... which has many cool add-ons including those that show you the PR (i recommend).