Hi there
A lot of this will come down to common sense and research. A couple of things I would point out:
Anchor text - Try and steer clear of exact match anchor text. It can trip spam filters if you have too much of that going on in your backlink profile. Try to make more use of branded anchor text. You can read morehere.
nofollow links - Don't disregard links because they are nofollow. They can be valuable and important, especially if that link is relevant to your business and helps expands your visibility to your audience. There's a great article here about nofollow.
PageRank - Google doesn't update this metric anymore. You can read more here. There are metrics elsewhere via OSE and more below that I list that you can find just as useful.
All in all - the best way to define a bad link is to use your own intuition and common sense:
-
Does this link help my website?
-
Is this link relevant to my website?
-
Would I trust this site (that's linking to me) if I landed on it?
-
Is the website or content in which I am being linked from topically relevant to my website?
-
If you check metrics - does anything about the metrics (domain authority, page authority,Majestic, SEMRush traffic/ranking data, etc) make me feel uneasy?
-
Are the links from directory templates? (example)
-
Inspect URLs with blatant spam words
-
Free
-
Porn
-
XXX
-
Submit
-
Directory
-
Paid
-
Links
-
URL
-
Sex
-
etc.
-
Check for multiple domains and URLs on the same IPs
-
This can usually show link farms or spam
Google views backlinks as who you associate with. If you hang out with shady people, you're probably a shady person. If you hang out with intelligent people, people probably think you're intelligent. Same applies for Google.
Hope this helps! Good luck!