Are they a stable longstanding company who deliver on their promises? Speak to their current client base, is their customer retention rate high? If their customers tend to remain with them this is a great indication that the company values its clients and the services they provide.
Do they have experience of working with sectors such as yours? Does the business and staff have good values? Can they provide strong references that are applicable to the solution they are offering? Do they offer case studies to support their sector knowledge?
Can you engage comfortably with them, are they approachable, open and honest? Do you share common views and do you generally get a good feel for their company?
Is the solution they are offering applicable to your company needs? Have they tailored the package to your needs or theirs?