how to check for an invalid PageRank?

It is a good question brought up in forum recently. Danny Sullivan at Search Engine Land just wrote an incredible authoritative guide to PageRank and highlights some points that still need to be reiterated: PageRank really doesn't matter in the grand scheme of things and it is only one measure of "importance" for your site but isn't updated all too often and thus shouldn't be considered heavily.

Still, a bunch of tools were offered to check the PageRank of a site, and Tamar Weinberg will be linking to them below:

Live PageRank: Checks your PageRank on all of Google's datacenters
Verify PageRank: Checks your PageRank and verifies its legitimacy
Fake PageRank Detection Tool: This one is currently not working, but the coder says he's trying to get it fixed so I'm linking to it in hopes that he will.
Check Fake PageRank: This tool also checks your PageRank and verifies if it is valid. Read More Here ...