How Do You Actually Vet A Software Development Company Before Committing?

I'm in the process of researching options for a custom software project and honestly finding it harder than expected to tell a genuinely capable software development company apart from one that just presents well during the sales pitch. Portfolios all tend to look polished, and everyone claims strong communication and technical expertise, so I'm trying to figure out what actually separates the good ones from the mediocre ones once a project is underway. So far I've been looking at past project examples, checking for relevant industry experience, and trying to get a sense of how they handle communication during the proposal stage. But I feel like there's probably a better way to vet this than just going off gut feeling and a few reference calls that are likely cherry-picked anyway. For those who've been through this process, what red flags did you wish you'd caught earlier, and what questions actually helped you separate a strong team from one that just talked a good game? Would really appreciate hearing what worked, and what didn't, from people who've actually gone through a full project from start to finish.