Start with domain experience, not the size of the portfolio. Request three or four engagements that sit close to your domain and your stack, which is better laravel or .net and then find out whether those engineers are still with the company. A serious vendor will introduce you to the people who would work on your project. Answers that name nobody at this stage almost always mean you are talking to a reseller.
The contract warrants a slower read than the pitch. A few clauses carry most of the weight: ownership of the code, non-disclosure, and notice periods and handover. All the work product must transfer guide to software development outsourcing you as it is paid for, along with designs, scripts and infrastructure configuration. Look closely at any clause that keeps framework code with the vendor, as that is often exactly the piece that locks you in.
Find out how the estimate was built. An honest estimate arrives with a list of assumptions, a breakdown per feature and a best case and a worst case. A fixed price is only reasonable when the requirements are stable and documented; when the scope is still moving the vendor adds a risk premium and you pay for uncertainty either way. Hourly billing shifts that risk to you, so it needs a sprint cadence, fintech software development demos and a budget cap.
How the work is run matters more than the number of developers. Find out how a new requirement enters the plan, who defines done and what the QA setup looks like. A well-run team will be able to demonstrate a live build at the end of each sprint. Clear, written acceptance criteria stay the only reliable protection against an argument at delivery time.
Before signing, think about the day you no longer need this vendor while the relationship is still good. Ask that the code repository sits on infrastructure you own from the beginning, and that a readme and architecture notes are kept current as the code changes. A partner who is comfortable with this says yes immediately; resistance at this point says most of what you need to know.