Well if its good designed blog with a lot of content and images,videos then look at their articles and see if most have only 1 ******** link, see which anchors are used promotional most time (buy targeted traffic, iphone 6 repair, seo australia..etc) you should notice if site is only publishing posts for links.. check if there are any relevant comments on their articles, so after some time you should be able to spot these guest post farms and better built PBNs right away. If you have ahrefs put the site and check links, organic traffic.. if its 0 organic keywords 0 traffic then most likely is pbn or guest post farm (but this not always right, i seen sites that have traffic and rank keywords but are obvious pbn's)