Start with proven experience, not the size of the portfolio. Request two or three case studies that sit close to your stack, and then ask who actually wrote that code. A solid partner will put you on a call with the tech lead. Evasive answers at this stage almost always mean you are talking to a reseller.
The agreement warrants a slower read than the pitch. A few clauses carry most of the weight: ownership of the code, non-disclosure, and exit terms and handover. All the work product has to transfer to you once invoices are settled, including designs, scripts and infrastructure configuration. Watch for any clause that leaves so-called reusable libraries outside the transfer, as this is frequently the part you cannot replace later.
Ask where their numbers come from. A credible estimate arrives with the assumptions behind it, a task-level breakdown and a best java development company case and a worst case. A fixed-bid deal is only reasonable when the requirements are stable and node.js development outsourcing documented; when the scope is still moving the provider adds a risk premium and you pay for it anyway. A time-and-materials model shifts that risk to you, so it needs a cap, regular demos and transparent reporting.
Process matters more than the number of developers. Ask what happens when the scope changes, who signs off on a feature and how quality assurance works. A team can walk you through running software rather than status reports. Written acceptance criteria stay your only real protection against endless rounds of rework.
Last, think about the day you no longer need this vendor while the relationship is still good. Ask that the code repository lives under your account from day one, and that documentation is written as you go rather than left to the end. A partner who is comfortable with this will agree quickly; resistance at this point tells you quite a lot.