Start with domain experience, not the number of logos on the website. Ask for three or four engagements that match your technology stack, and then ask specifically who actually wrote that code. A serious vendor is happy to connect you with the tech lead. Answers that name nobody at this stage generally mean the delivery team is not the team you were shown.
The paperwork deserves more attention than the sales deck. Three sections matter more than the rest: intellectual property assignment, the NDA, and termination and handover. All the work product should transfer to you once invoices are settled, along with documentation, angular for enterprise applications pipelines and deployment scripts. Be careful with wording that leaves framework code outside the transfer, since this is frequently the dependency that makes switching painful.
Find out how the estimate was built. An honest estimate comes with the assumptions behind it, a breakdown per feature and an explicit range. A fixed-price contract only makes sense when the scope is genuinely frozen; otherwise the provider pads the number and you fund the buffer regardless. Time and hire eas developer materials moves the risk back to the client, so it demands visible weekly reporting and top flutter development companies a spending cap.
Process beats the number of developers. Ask what happens when the scope changes, who signs off on a feature and what the QA setup looks like. A mature team should be able to show you a working build every one or two weeks. Written acceptance criteria remain the only reliable protection against endless rounds of rework.
Last, think about the day you no longer need this vendor while the relationship is still good. Require that the source repository stays on infrastructure you own from the first commit, java development services and that the documentation is refreshed in every sprint. A vendor with nothing to hide will agree quickly; resistance at this point reveals most of what you need to know.