When searching for Link Building Services, one should look for examples of proven success. A solid link building service will make past successes available for all to see.
If the service claims to be ranked number one in Google for “widgets,” be sure to follow it up. Search “widgets” in Google and see if the service is truly number one. If it is, then that link building service might be a good choice.
Any link building service should be honest about how links are obtained. There are straight foreword ways of building links (observing search engine guidelines) and there are ways that are considered to be “black Hat” (not observing search engine guidelines).