I recommends, before to pay for the work, the link-company should sent you the list of the domains where they will put your links, and you should check:
1. The IP where the linking domain is hosted (prevents link farms or other networks, Google knows that).
2. The owner of the domain "attention with hidden registrants", several companies collect hundred of blogs and forums to use for link building and of course Google know that.
3. These links should be included in a natural way (no all links in the same week).
Be carefully, last year I bought a pack of 50 links (PR/3/4/5/6) and for my surprise, no one improvement in ranking ... SO Google knows these people.
Hope it helps