I use: http://www.verifygooglepagerank.com/ Also you can check +40 google datacenters. So you can see if your pagerank will change because google update these datacenters on different times.
I use fire fox Google tools bar. It shows me the PR of each page I am visiting also I have SEO quake fire fox plug-in that can show the PR of all sites in a Google search result.
I'd not say the toolbar is very accurate though. I always end up getting the result from a dodgy datacentre that shows a different PageRank. :/