Look first at domain experience, not the length of the client list. Ask to see a couple of case studies that match your domain and your stack, and then ask who actually wrote that code. A serious vendor wordpress vs laravel will introduce you to the engineers. Answers that name nobody at this stage usually mean the demo work came from somewhere else.
The agreement needs a slower read than the pitch. Three clauses do most of the work: ownership of the code, difference between flutter and react native non-disclosure, and termination and handover. Every artifact has to transfer to you once invoices are settled, including documentation, pipelines and deployment scripts. Watch for any clause that keeps framework code with the vendor, since this is frequently exactly the piece that locks you in.
Find out how the estimate was built. An honest estimate arrives with a list of assumptions, a task-level breakdown and an explicit range. A fixed-bid deal only makes sense when the requirements are stable and documented; otherwise the vendor pads the number and you pay for uncertainty either way. software development hourly rate billing moves the risk back to the client, so it needs visible weekly reporting and a spending cap.
Process matters as much as team size. Ask how a new requirement enters the plan, who writes the acceptance criteria and what the QA setup looks like. A well-run team should be able to show you a live build at the end of each sprint. Acceptance criteria in writing are the practical protection against endless rounds of rework.
Finally, consider the end of the engagement at the start rather than at the end. Insist that the repository lives under your account from the first commit, and that a readme and architecture notes are kept current as the code changes. A partner who is comfortable with this accepts it without argument; hesitation here says most of what you need to know.
