Ask HN: How do you hire an app development firm?

A college friend based in LA asked me how he can find a firm to totally redesign and rebuild his company's web app. They're running on 90s design and technology and need a complete overhaul.

How can he figure out what good app development shops are out there? I suggested he attend some local dev meetups and ask them. But, I also wanted to check here.

Know of any good app dev firms or places to find them?