Begin with relevant experience, not the size of the portfolio. Request a couple of engagements that resemble your technology stack, and then ask specifically whether those engineers are still with the aws web development company. An honest provider will introduce you to the engineers. Vague answers at this stage almost always mean the demo work came from somewhere else.
The paperwork needs more attention than the sales deck. Three sections matter more than the rest: intellectual property assignment, non-disclosure, and notice periods and handover. Everything produced should transfer to you once invoices are settled, together with documentation, pipelines and deployment scripts. Look closely at language that leaves framework code outside the transfer, since it is usually exactly the piece that locks you in.
Ask how they estimate. A credible estimate comes with a written set of assumptions, a breakdown per feature and a range rather than a single number. A fixed-bid deal works only when the requirements are stable and documented; in any other case the supplier prices the risk in and angular enterprise application you pay for it anyway. Hourly billing moves the risk back to the client, so it demands a sprint cadence, demos and a budget cap.
How the work which is better rest or graphql run beats headcount. Ask what happens when the scope changes, who signs off on a feature and how quality assurance works. A mature team should be able to show you a live build at the end of each sprint. Acceptance criteria in writing are your only real protection against endless rounds of rework.
Last, think about the handover at the start rather than at the end. Require that the code repository sits on infrastructure you own from the beginning, and that documentation is updated as part of the work. A vendor with nothing to hide says yes immediately; a long negotiation over it reveals most of what you need to know.