Start with domain experience, not the length of the client list. Ask for three livewire or react four case studies that match your technology stack, and then ask specifically which engineers actually built it. An honest provider will put you on a call with the tech lead. Vague answers at this stage almost always mean the delivery team is not the team you were shown.
The paperwork deserves more scrutiny than the proposal. Three clauses do most of the work: assignment of intellectual property, confidentiality, and termination and handover. All the work product has to transfer to you as it is paid for, including designs, scripts and infrastructure configuration. Look closely at any clause that keeps reusable components in the vendor’s hands, as that is often exactly the piece that locks you in.
Ask where their numbers come from. A credible estimate is accompanied by a written set of assumptions, a breakdown per feature and an explicit range. A fixed price works only when the specification is complete; otherwise the supplier adds a risk premium and you pay for uncertainty either way. A time-and-materials model shifts that risk to you, so it demands a cap, nextjs development services regular demos and transparent reporting.
How the work is run matters as much as headcount. Find out how change requests are handled, who defines done and what the QA setup looks like. A mature team should be able to demonstrate a working build every one or hire remote flutter developers two weeks. Acceptance criteria in writing remain the only reliable protection against the it-was-never-in-scope conversation.
Last, plan for the end of the engagement while the relationship is still good. Require that the code repository lives under your account from the first commit, and that the documentation is refreshed in every sprint. A partner who is comfortable with this accepts it without argument; a long negotiation over it tells you quite a lot.