1) check anchors, something viagra, cialis or anything foreign should raise alarms, but if you see something foreign translate in google and see if it makes sense. If you see anchors that are unrelated to the previous website, that is an alarm. Also, if you see a related/unrelated anchor that has a very high percentage then it is a sign that the original owner was over optimizing anchors.
2) check hosting history, if the domain was dropped multiple times, pay attention to hosting providers, were they foreign? do you see an .jp or Chinese hosting?
3) Always check archive, from the day the domain was registered for the first time, if it was dropped and re-registered, pay special attention to those dates and see if the website changed, was it a PBN? did someone already re-purpose it? etc... If archive is missing for a year or so, then fuck it, don't use that domain.
4) Check backlinks, don't rely on metrics, look for contextual do-follow links, or links from authoritative sites in the niche or editorial links.
5) I like to get my hands on indexed domains. search site:domain.com in google and see if it is indexed, if it is, look for content, does it make sense? if someone re-purposed it then you would see irrelevant or something out of the ordinary. Alternatively, look at the 'top pages by links' in ahrefs and if you see something out of the ordinary, just move on.
There's probably more stuff you can do, but this is what I do to thoroughly vet a domain. You can be more flexible with PBN's, if you're using it as your money site, then you need to make sure that everything lines up perfectly.
Good luck!
