A quote that comes back within a day is a warning, not a service level. A competent team will come back with a list of questions: about users and volumes. A provider that quotes before understanding the scope is simply working from a template, and symfony vs spring the gap becomes a change request later — and custom react native development you will pay for it.
Watch for a gap between the engineers on the sales call and the people who will code. Request specific people rather than roles in the contract, with a clause covering replacement. A vendor that talks only about roles and never names people is reserving its own flexibility at your cost.
Ask for the source repository from the start. A provider that delivers nothing between demos expects you to trust a black box. Regular commits and pull requests tell you who is really on the project far better than any status report. This extends to the automated test suite: legacy code maintenance services if nothing runs automatically, promises about quality remain unverifiable.
Vague contract language around code ownership is never a formality. The contract needs to state in plain terms that the code, designs and documentation transfer to the client on payment. Look too at the jurisdiction and the milestone terms: a request for most of the money up front with nothing due in return for weeks takes away the only leverage you have.
Lastly, look at how they communicate. Ask what overlap the teams will share with your working day, which named person answers questions and on what response times. A few hours of overlap is normally sufficient; no overlap stretches each small question into a lost day. Careless writing in the early emails will not improve later.
