Begin with proven experience, not the size of the portfolio. Ask to see three or four engagements that match your technology stack, and then ask which engineers actually built it. A solid partner will introduce you to the engineers. Evasive answers at this stage usually mean the delivery team is not the team you were shown.
The agreement needs more attention than the sales deck. Three sections matter more than the rest: ownership of the code, non-disclosure, and termination and handover. Everything produced should transfer to you once invoices are settled, including source code, designs and infrastructure as code. Watch for language that leaves reusable components with the vendor, since it is usually exactly the piece that locks you in.
Find out how the estimate was built. An honest estimate is accompanied by the assumptions behind it, a breakdown per feature and a best case and a worst case. A fixed-price contract only makes sense when the specification is complete; when the scope is still moving the vendor native app development prices the risk in and you pay for uncertainty either way. A time-and-materials model moves the risk back to the client, so it needs visible weekly reporting and a spending cap.
Process matters more than team size. Find out how a new requirement enters the plan, who defines done and how quality assurance works. A team should be able to show you a live build at the end of each sprint. Written acceptance criteria are the only reliable protection against an argument at delivery time and materials contract.
Finally, plan for the end of the engagement before it becomes urgent. Require that the source repository stays in your organisation from the beginning, and that the documentation is refreshed in every sprint. A provider confident in its own work will agree quickly; resistance at this point reveals quite a lot.