Start with relevant experience, not the size of the portfolio. Ask for a couple of engagements that resemble your stack, and then find out which engineers actually built it. A solid partner is happy to connect you with the engineers. Evasive answers at this stage usually mean you are talking to a reseller.
The agreement deserves a slower read than the pitch. Three clauses do most of the work: ownership of the code, confidentiality, and termination and handover. Every artifact must transfer to you as it is paid for, together with documentation, pipelines and deployment scripts. Be careful with wording that leaves so-called reusable libraries in the vendor's hands, rust development company because that is often exactly the piece that locks you in.
Find out how the estimate was built. A credible estimate comes with a list of assumptions, a breakdown by feature or module and a range rather than a single number. A fixed price works only when the specification is complete; in any other case the supplier pads the number and you pay for uncertainty either way. Hourly billing puts the risk on your side, so it requires visible weekly reporting and a spending cap.
The delivery process matters as much as team size. Establish how change requests are handled, who defines done and how testing is organised. A well-run team should be able to show you a working build every one or two weeks. Written acceptance criteria stay your only real protection against the it-was-never-hire developers in usa-scope conversation.
Before signing, plan for the handover at the start rather than at the end. Ask that the code repository lives under your account from the beginning, and that documentation is updated as part of the work. A vendor with nothing to hide will agree quickly; resistance at this point tells you a great deal.
