First, what is page rank (PR)? PR is a link analysis algorithm used by Google to assign a value of 0-10. There are a lot of mysteries behind what goes into and how the algorithm weights certain factors, but it basically revolves around links back to your website. The higher the page rank of the site that links to you the more value that links receives as benefits your PR. Simply, page rank is the perceived quality of your website, the more back links you have and back links from higher PR sites the better.
Page rank should not be confused with SERP (Search Engine Results Page) ranking. SERP is your page ranking for a set of searched keywords. While PR does play a factor is your SERP, it also takes into account site title and meta tags. SERP can be looked at as actual relevance to a search term, compared to PR that is perceived value or popularity.
How do you determine what your PR is? Google is constantly crawling and factoring page rank constantly. It is computed to the hundredth decimal place. The value made available to the public is the rounded up / down integer of this number and there is not a set schedule when Google updates the publicly available number. A Google search for page rank checker will turn up plenty of results. I tend to use a handful to check accuracy and make sure no variations.
You now have a PR assigned to the website you looked up. However, it may not be valid! How can it not be valid? The page rank of a website can be manipulated by a flaw in the system. If a website is redirected to a website of higher page rank the website will inherit the PR of the website it directs to. One-way to check the validity is to do a search for the website in question. If the results turn up a completely different domain the PR is not valid. Of course, there are also tools available to check for validity.
If you are anxious to know what your PR will be once Google updates the publicly used numbers you can estimate the new value using a keyword prediction tool that analyzes current back links and traffic.