I just wanted to add to what Brendan said, I would start by searching in Google by using keywords you want to rank for and then keep searching, using keywords that are thematically relevant to your site.
Using the SEOQuake (see below for optimal settings) and SEOMoz toolbar in the Google SERP will give you an indication of whether or not you even want to view the site (If PageRank is 4, DA is >50, etc, etc.). Go 100 results deep, the first two pages will likely be competitors that wouldn't link to you anyway.
Again, like Brendan said, if the site is ranking for a relevant keyword then it's safe to assume Google views that page/site favorably!
Once you have looked at the site and made sure there aren't any irrelevant (or paid) links on the site and it passes your metrics be sure to check the inbound links to the site and the page if it's an inside page.
It will depend on your industry but generally I won't look at an inside page if ti has less than 10 links to it. I've seen too many PR4s lose page rank in the next update because it was only getting juice internally, so the inbound links must be from external sources.
I also question the tlds of the inbound links. I will usually 'head for the hills' if I see .ru, .cn, .id, etc you know the drill.
Hope that helps.
**For SEOQuake, when I use it turned on in the SERP I have it show Google PR, Indexed Pages, and I used to show the Yahoo Link and Link Domain counts
Make sure you have it set for 'parameters by request' otherwise you'll timeout in Google in no time.