Start with domain experience, not the length of the client list. Ask to see a couple of projects that match your stack, and then ask who actually wrote that code. A solid partner will put you on a call with the tech lead. Evasive answers at this stage generally mean the demo work came from somewhere else.
The contract needs more attention than the sales deck. Three clauses do most of the work: assignment of intellectual property, the NDA, and exit terms and handover. All the work product should transfer to you once invoices are settled, together with designs, scripts and infrastructure configuration. Be careful with language that leaves reusable components with the vendor, since this is frequently the part you cannot replace later.
Ask where their numbers come from. A serious estimate arrives with a list of assumptions, a task-level breakdown and a best case and a worst case. A fixed-bid deal works only when the requirements are stable and documented; in any other case the vendor adds a risk premium and you pay seo agency for saas uncertainty either way. A time-and-materials model shifts that risk to you, so it needs a cap, top react native development companies regular demos and transparent reporting.
How the work is run matters as much as the number of developers. Ask how a new requirement enters the plan, who defines done and what the QA setup looks like. A team should be able to show you a live build at the end of each sprint. Acceptance criteria in writing stay your only real protection against endless rounds of rework.
Before signing, consider the day you no longer need this vendor while the relationship is still good. Require that the source repository lives in your organisation from the first commit, and that hire a development team readme and architecture notes are kept current as the code changes. A vendor with nothing to hide says yes immediately; hesitation here tells you quite a lot.