Firstly, reputation. A reliable vendor should have good reviews on independent services (e.g. Clutch) and, ideally, references from other current and past clients. The reputation is a safeguard in itself: as it is a valuable asset that attracts customers, companies will strive to maintain it. Moreover, it is proof of the vendor’s quality.
Secondly, specialized knowledge. This goes beyond understanding the domain and its standards (HIPAA, HL7, GDPR, IMS Global, etc.). Knowledge includes having proper processes inside the company and regarding projects, approaches to key project milestones (initiation, delivery, support, etc.), having ready-to-use components for specific software types, and more. You can find out about that by asking the right questions.
Finally, convenience. An outsourcing vendor shouldn’t make your life harder. On the contrary, they should deliver you success like a conveyor belt. And it is largely on them to organize smooth communication and guide you through the process, that’s what you’ve hired them to do.