You want to make sure that your consultant has experience to back up their claims. They should be able to cite specific instances of working with problems that you may face and often they will have certifications as well.
Another important factor is customer satisfaction. You want to get references from a company to find out what their former clients have to say about them. If their customers are happy and can back up their claims, then you should feel no hesitation signing a computer support contract.
For more on separating the wheat from the chaff when it comes to computer support contracts, read the full article (link above).
Blogged by Computer Consulting 101