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Telling wheter links are "good links" is very important to anyone who wants to get into Search Engine Optimization.

There are a number of things you can do, ranked in order of importance:
- Check if the links are "direct", some directories or CMS use redirects. If the link is a direct link (good) you can tell by hovering the link and reading the url in the bottom left of your browser, this url should be the addres the website that is linked to. If the site in question is using redirects, you will see the link will be something like http://www.hissite.com/somescript.ph...ite.com?blabla.
Some scripts use javascript to place other text there, like google adsense, they don't use direct links but they link to their tracking software and then redirect you the the advertisers's site. When you hover an adsense link, a javascript will interfere and display go to -advertiser.com- or something like that.
-Check if the site uses nofollow, some sites add nofollow attributes to links, you can check by viewing source and doing a search for: rel="nofollow". Using nofollow on a link will make a search engine ignore this link.

Here is a cool firefox hack from Matt Cutts (he works for Google) that allows you to see nofollow links in firefox: http://www.mattcutts.com/blog/seeing-nofollow-links/

-make sure the page is indexed by at least one major search engine, do this by searching the url in for example google.

-Make sure the page that links to you is linked to by other pages. You should use Yahoo to check backlinks on the page, like so:
http://search.yahoo.com/search?p=lin...&cop=&ei=UTF-8

Sometimes you'll see that the link page you are trying to add a link to, is only linked to by other pages from the guy his site. In order for this link page to be worth anything wrt Search Engine Optimization, the rest of the guys site needs to be linked too by other site, you can check this by using the yahoo linkdomain command:
http://search.yahoo.com/search?p=lin...ab-web-t&x=wrt

Do not use www and http with this search.