Look for professionals.
Look for companies that have been around for a while and that have a proven track record. Is there a portfolio on their web site, are there genuine customer testimonials. Whilst start ups can be equally good, if the business is established, they have obviously established a solid business model and are likely to be around for some time to come. Once you’ve established a great relationship, you don’t want to be looking for an outsourcing partner all over again in a few weeks time because the one you chose set their rates too low and went out of business.