There are various ways to check whether a domain has a fake page rank and the simplest one I use is Google's info operator
Just type: info:www.domain.com into the search bar in Google and if the results show a different domain name then it is probably a fake.
This usually occurs because the domain name has been redirected to another domain with higher page rank.
See this fake PR6 below for a domain (www.voiceofthediabetic.org) – currently on 7 day auction sale with Godaddy.com :
It's very disappointing to buy a fake and a lot of these get sold every day, both intentionally and unintentionally on the sellers behalf.
Nathan is looking to include this filter in the domain software we are developing.
I keep calling it “the software we are developing” because there is a little debate going on right now around the name of it. Watch this space!