Start with relevant experience, not the size of the portfolio. Request two or three case studies that resemble your stack, and then find out which engineers actually built it. 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 contract needs a slower read than the pitch. Three clauses do most of the work: dedicated teams vs freelancers intellectual property assignment, the NDA, and termination and handover. Everything produced must transfer to you once invoices are settled, together with designs, scripts and offshore software development infrastructure configuration. Watch for language that keeps reusable components outside the transfer, since it is usually the part you cannot replace later.

Find out how the estimate was built. A credible estimate arrives with a list of assumptions, a breakdown per feature and a best case and a worst case. A fixed-price contract is only reasonable when the scope is genuinely frozen; in any other case the vendor adds a risk premium and you fund the buffer regardless. Hourly billing shifts that risk to you, so it needs a cap, regular demos and transparent reporting.

Process beats the number of developers. Ask how a new requirement enters the plan, who defines done and how testing is organised. A team can demonstrate running software rather than status reports. Written acceptance criteria stay the only reliable protection against an argument at delivery time.

Before signing, plan for the handover before it becomes urgent. Require that the code repository lives under your account from day one, and that a readme and architecture notes are kept current as the code changes. A vendor with nothing to hide says yes immediately; hesitation here says most of what you need to know.