Pingback - spam of legit?

Here is a dilema I have. I have received some pingbacks in the past and I'm not completely sure on their legitimacy. Rather, I'm not sure if they're truely sites that just like the content so thing send a shout out to the article or if they're just trying to get more links out there to imporve their search engine rankings.

So, for those bloggers out there who are 'gurus' ... how can you tell the difference between a legit ping and spam?