Start with domain experience, not the number of logos on the website. Ask for three or four case studies that resemble your technology stack, and then find out whether those engineers are still with the company. A solid partner is happy to connect you with the tech lead. Answers that name nobody at this stage almost always mean the demo work came from somewhere else.
The agreement needs a slower read than the pitch. Three clauses do most of the work: assignment of intellectual property, confidentiality, and notice periods and handover. Every artifact must transfer to you on payment, including source code, designs and infrastructure as code. Be careful with any clause that leaves so-called reusable libraries in house team vs outsourcing costs the vendor's hands, since that is often the part you cannot replace later.
Ask where their numbers come from. A serious estimate comes with the assumptions behind it, a task-level breakdown and an explicit range. A fixed-bid deal only makes sense when the requirements are stable and documented; when the scope is still moving the supplier adds a risk premium and you pay for it anyway. Time and materials moves the risk back to the client, so it demands visible weekly reporting and a spending cap.
How the work is run matters as much as headcount. Ask how a new requirement enters the plan, who signs off on a feature and how quality assurance works. A team will be able to show you running software rather than status reports. Written acceptance criteria remain the practical protection against an argument at delivery time.
Before signing, consider the end of the engagement at the start rather than at the end. Ask that the source repository sits on infrastructure you own from day one, react native developer for hire and that documentation is written as you go rather than left to the end. A vendor with nothing to hide will agree quickly; hesitation here says most of what you need to know.
