enter to google wirte : cache:www.domainname.com Press enter you will see a page. If the url of cache is same wtih the domain name that you entered it's valid else it's fake.
Check this out..but dont try.. http://seoblackhat.com/2005/10/20/fake-any-google-page-rank-with-user-agent-redirect/
I do not know if it is ok tell how to fake pagerank, but this is not actually about fulling google toolbar it is more about how google works. Those who fake pagerank take adventage of the fact that google updates their PR once a 3 months and that the PR is passed with permament redirection