How can I tell if incoming links are the spammy kind?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Banyan Tree Healing Center: So in my web master tools account I can see all the incoming links to my website and there are a lot. ; How can I tell if any of them are the spammy kind? ; Is this harmful. ; And if they are, what should I do about it?
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  Dmitry Grunt: If you didn't receive warning from google about unnatural links and site get normal position in serp and traffic, its ok, dont worry about links.
  
  If traffic fall down in short  ;time, better to check all links and disavow links from low quality sites.
  
  links on pages with text "buy viagra, download porn" etc must be disavowed 100%.
  
  PS GWT didt show ALL links, for deep research you can use ahrefs and magesticseo.
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  Banyan Tree Healing Center: +Dmitry Grunt
  Thank you, this is reassuring. ; ; I saw some very strange links I didn't recognize, but no warnings at all from google. ; ; My site may be suffering for other reasons perhaps. ; ; ; Is there any particular way to discern which are low quality sites?
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  Dmitry Grunt: +Banyan Tree Healing Center ;bad sites: sells links, has spammy content, has very low positions in serp  ;> 100.
  
  and if you see very poor design, site with free template, site on free hosting check it twice.
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  Adam Heaton: I'd like to add, just because you haven't got a penalty now doesn't necessarily mean you won't get one in future. If there are any links you are worried about, I would either have them removed or disavow them so they don't affect your website in future. This of course depends on how paranoid you would like to be about your websites rankings.
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  Tony McCreath: By suffering do you mean you did well in the past and have seen a drop. Or have you never done well?
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  Banyan Tree Healing Center: +Tony McCreath yes I used to rank well but have been demoted to page 2 and sometimes page 3. I have a few issues going on. And just trying to explore all possibilities. 
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  Banyan Tree Healing Center: +Adam Heaton I am feeling a little paranoid. So I may have to explore this deeper.