I would usually check the sites from where I will be building my links from. Sample, a site must: have higher PR BUT not necessary since not all sites with high PR are good/clean sites, have quality contents, regularly update its site, don't have spam ads, don't have adult or porn links or sites like that, have active readers and you'd know that in the blog comments section per posting. Any site that seems to function decently and doesn't seem like a spam site, that's a good site to build your links from.