What are some red/green flags when hiring a software company for a project?

I am trying to find a software company that can create a web app that I envisioned. It is an app that visualizes data. It has some complex details that I want to make sure get done right. Currently I am on Upwork talking to software companies, not solo freelancers.

I'm wondering if there are any red flags I should look out for when I talk to them in zoom meetings? Or if there are good signs? I am new to this so It is hard to gauge what hints to a good company vs a bad one. besides the Upwork reviews, I have nothing to go on.

Any advice is greatly appreciated!