Start with the problem, not the technology. A good software development company asks about your customers, workflow and budget before recommending a stack.
Check real work and experience. Ask to see live projects, how long the team has been building, and who will actually write your code.
Look at process and communication: written scope, milestones, staging links and a clear support plan after launch.
Finally, compare ownership terms. You should own your source code, domain and hosting accounts from day one.