Can anyone tell me how is a quality backlink?
Are there tools to check that or i can do manually checks?
I think you meant to ask "What is a quality backlink?"
Here are the things I look at to give me an idea of how much a link from a page/website might be worth:
* Page Authority of the page as measured by Moz toolbar
* Domain Authority of the domain as measured by Moz toolbar
* PageRank (MozRank) of the page as measured by Moz toolbar and/or SEO Quake toolbar
* Domain Trust as measured by Moz
* Trust Flow as measured by MajesticSEO
* Citation Flow as measured by MajesticSEO
In all of the above situations, more is better, especially Trust flow, Page Authority and Domain Authority
I would also consider this:
* Is the link rel="nofollow" or not (If the link is rel="nofollow" then consider it worthless for SEO)(still might want the link though)
* How many outbound links are there on the page in total (less links on the page means more PageRank passes through my link)
* Are there any spam links on the page (I want zero spam links on the page)
* Where would my link be positioned on the page? (preferably above other links and preferably in the highest part of the heatmap on the page)
* Can I get the anchor text I want (hopefully yes, but I generally disregard whether or not I can get the anchor text that I want)
* Will this link remain permanently or at least semi-permanently? (I generally don't waste my time on temporary links)
* Will the link look natural or not? (if not then I would second guess whether to get it)
A lot of people worry about relevancy of links and consider a highly relevant link to be higher quality. Personally I prefer a relevant link, but for SEO purposes I don't care if it is relevant or not.