Spam: thin site, lacks original content, few pages indexed, crawled infrequently, ranks for a small selection of keywords, weak backlink profile
Authority: many pages indexed, thick site, loads of original content, crawled frequently, ranks for a variety of keywords, diversified backlink profile
Page Rank used to be a strong indicator of this, but currently a better indicator is how often your site is crawled. That's why one authority link can do worlds more for you than hundreds (or thousands) of spammy/low quality links.