Review on Flawless by Noah Smith

It's free! And it works great with Xcode, so you can use your existing workflow to do QC checks before deploying any changes or code into production (or even just testing new features). I'm using this tool in conjunction with TestFlight builds that are deployed from my own server; otherwise, there would be no way of checking if things look good across all devices & simulators used during development/testing phases - which is very important when dealing with large teams where everyone has their hands full already without having time to play QA engineer.