Start with domain experience, not the size of the portfolio. Ask to see two or three engagements that sit close to your stack, and then ask specifically whether those engineers are still with the offshore software development company. A serious vendor will introduce you to the tech lead. Answers that name nobody at this stage almost always mean the delivery team is not the team you were shown.
The contract warrants more scrutiny than the proposal. A few clauses carry most of the weight: next js development agency ownership of the code, non-disclosure, and exit terms and handover. Everything produced should transfer to you on payment, along with designs, scripts and infrastructure configuration. Look closely at wording that leaves so-called reusable libraries outside the transfer, because it is usually the dependency that makes switching painful.
Ask how they estimate. A credible estimate arrives with a list of assumptions, a breakdown by feature or module and a range rather than a single number. A fixed-price contract works only when the scope is genuinely frozen; in any other case the vendor adds a risk premium and you pay for it anyway. Time and materials puts the risk on your side, so it demands a sprint cadence, demos and a budget cap.
The delivery process matters more than the number of developers. Ask how change requests are handled, who signs off on a feature and top react native development companies what the QA setup looks like. A well-run team will be able to walk you through a working build every one or two weeks. Acceptance criteria in writing stay the practical protection against the it-was-never-in-scope conversation.
Finally, plan for the end of the engagement while the relationship is still good. Require that the repository sits under your account from the beginning, and that documentation is updated as part of the work. A partner who is comfortable with this accepts it without argument; resistance at this point says a great deal.