I find the best way to check out a company is to ask for referrals. Although the prospective vendor is only going to give you customers that are happy with their service, calling those companies & asking questions that pertain to your circumstances will give you clues on which vendor is right for your specific needs.

Ask for referrals in different sectors (manufacturing, retail, distributor, etc) to try to match one to your sector. If you're in retail, your needs will be different than a manufacturer.