Start with domain experience, not the length of the client list. Ask for three or four engagements that match your technology stack, and then ask specifically whether those engineers are still with the company. A serious vendor is happy to connect you with the engineers. Answers that name nobody at this stage usually mean you are talking to a reseller.

The paperwork warrants more scrutiny than the proposal. A few clauses carry most of the weight: ownership of the code, confidentiality, and termination and handover. Everything produced should transfer to you on payment, together with source code, designs and infrastructure as code. Look closely at language that keeps framework code in the vendor's hands, since that is often 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 range rather than a single number. A fixed-price contract only makes sense when the specification is complete; in any other case the supplier adds a risk premium and you pay for alpine js vs livewire uncertainty either way. A time-and-materials model shifts that risk to you, so it needs a sprint cadence, demos and a budget cap.

How the work is run beats the number of hire developers in uk. Ask how a new requirement enters the plan, who signs off on a feature and how testing is organised. A team should be able to show you running software rather than status reports. Acceptance criteria in writing are the only reliable protection against an argument at delivery time.

Last, plan for the day you no longer need this vendor before it becomes urgent. Insist that the repository sits under your account from the beginning, and that the documentation is refreshed in every sprint. A provider confident in its own work will agree quickly; hesitation here tells you most of what you need to know.