https://www.reddit.com/r/SomebodyMakeThis/ https://www.reddit.com/r/AppIdeas/ https://www.reddit.com/r/apps/ https://www.reddit.com/r/webdev/ InVision you would use before hiring someone, this is an easy to use design / design flow service that syncs with photoshop that you would share with a developer. (Twitter, Uber, and Netflix I Know use this)
To gadge how big/well known a developer/developer company/web service company is see how big and how many followers their twitter has, the webdev community kinda trains you to use this as a tool and find their github.com page to gadge what other projects they've done, here you;'d check To communicate with developer while in project is in development you should use (or expect them to use) Slack.com or the chat feature InVision uses (assuming you're using InVision) https://www.invisionapp.com/