Begin with proven experience, not the length of the client list. Ask for two or .net vs laravel three projects that match your stack, kotlin development services and then find out who actually wrote that code. An honest provider is happy to connect you with the tech lead. Vague answers at this stage usually mean the delivery team is not the team you were shown.
The agreement deserves more attention than the sales deck. A few clauses carry most of the weight: assignment of intellectual property, confidentiality, and termination and handover. Every artifact has to transfer to you once invoices are settled, along with designs, scripts and infrastructure configuration. Look closely at language that keeps reusable components outside the transfer, since that is often the dependency that makes switching painful.
Find out how the estimate was built. A serious estimate is accompanied by the assumptions behind it, a task-level breakdown and an explicit range. A fixed price is only reasonable when the specification is complete; in any other case the supplier pads the number and you pay for uncertainty either way. A time-and-materials model puts the risk on your side, so it demands visible weekly reporting and a spending cap.
Process matters as much as headcount. Establish how a new requirement enters the plan, who defines done and how quality assurance works. A team will be able to demonstrate a working build every one or two weeks. Acceptance criteria in writing stay the practical protection against endless rounds of rework.
Finally, think about the day you no longer need this vendor at the start rather than at the end. Require that the source repository lives on infrastructure you own from the first commit, and that the documentation is refreshed in every sprint. A vendor with nothing to hide accepts it without argument; resistance at this point says a great deal.