You probably wouldn’t by a new TV or a new car without reading reviews and getting feedback from other people that have bought the same item and link building services should be no different. Although you may not be able to speak to people direct to ask about their experiences with a link building services company you should be able to get an idea of the work that they have done in the past.
Most companies will have a portfolio that they will be happy to show you. They should have an idea of where the company was when they started and how much they are achieving now that they have undertaken link building services and SEO packages. This should give you an idea of the type of projects they have worked on in the past and if they have what is needed for you and your company.
If a company doesn’t have this available for you then you should be very wary of listening to their claims of what they can do and have achieved. You really do need to see some level of experience and past work in order to be able to gauge the quality of the work that they offer for you.