Just use your head. Check:
If the top placed sites are "authorities" you know, the ones that have a list of pages underneath their G listing and the site is years old and has a billion backlinks, then forget it and target another site instead.
I always use this formula...
If the site I am targeting has a similar domain age to mine, if it has no more than approx. 5k Yahoo links and it doesn't have the keyword in its domain name and it's not an authority site, then it's fair game for me.
I always use the SEO Quake add on for firefox which can give you all of that info and more.