I tend to look at the PageRank and then look at the current backlinks to see if they support the PageRank. When you see a PR5 domain with two PR1 backlinks and a PR2 backlink, you know that a PR drop is coming for that domain.
The "fake pagerank detection" tools never seem that reliable to me, so I do this manually.
Yahoo is a lot better at reporting backlinks than Google, unless you can convince the seller to upload your Google Webmaster Tools key file to the web site.
Backlink Watch and Smart PageRank are front-ends to Yahoo search. They have interfaces designed for just this task, but they have lost a lot of their usefulness since Google blocked them from getting PageRank data.
I'm a Unix guy, so I just take the list of backlinks and run it through a shell script that shows me the PR of every backlink. (I think I got this script from shoemoney)
my $pr = WWW::Google::PageRank->new();
my $url = shift;
my $rank = $pr->get($url);
print "PageRank: " . $rank . " " . $url . "\n";