Ask for references and see past work. Make an overview of the work the partner has done and talk to current clients to confirm that they trust the company and are happy with the results.
It is advisable to visit the client, see the impact of outsourcing and find out how it’s working and what people think. Visit the company’s offices, speak with the people and see sample projects. It is good practice to visit a partner several times a year. This not only lets you see the work being done, but also strengthens the relationships.
You may want to start off with a small pilot project and tie payment to project deliverables.
You should not only look for an outsourcing company, you should also look for reliable people with a good track record.
Also, try to interview the technical team that will do the work. Many vendors have great sales teams, but poor execution talent.
Look at potential partners who either have experience in your industry, or manage other customers of around the same size.