What is link building? It is about promotion of your website and getting as many backlinks as possible. Links are a measure of popularity of your site. Each link is some kind of vote for your site, links are also the essential part of the PageRank algorithm used by Google to evaluate each web page according to its relevancy. This means that most of the SEO activities are focused to get backlinks from relevant pages.
Of course, all the links do not have the same value. The first link property that plays an important role in link building is the “nofollow” attribute. This attribute tells search engines not to follow this link (to discover new pages) and not to take it into account for PageRank calculation (this applies only to Google). The main purpose of this attribute was to mark some external backlinks in blog comments to be less attractive to reduce spam. However, this attribute is now used all over the web without any good reason. This means that many websites are using it for all outgoing links. This makes no sense because links are the essential part of the web which is in fact a collection of linked documents. Therefore, most backlink strategies are focused to get normal backlinks without this unnecessary attribute.
The second important property of the link is the anchor text. This is the text that actually points to another page. This text actually tells to search engines and human visitors what is the content of the page the link points to. This means that it is important not to use irrelevant text like “click here” or “this page” for anchor text.
Another important consideration is the location of the link. Links on relevant pages with similar content are much more important than links on pages with no content or on pages with many different links. This means that the goal should be to get most of the backlinks from similar and relevant pages.
But all backlink building strategies should not create “clean” links exclusively. There should also be some nofollow links and links with common anchor texts. This will reflect natural links as they are created without any intervention.
Now that you know how links should look like you can analyze the quality of backlink building service. This can be done by using some SEO tool which analyzes backlinks or by simply using Yahoo site explorer to discover backlinks to particular page and them check them manually. You should pay attention to the number of links, their location (relevant pages or not), anchor text distribution (avoid repeating the same words) and ratio between normal and nofollow links.