There is a little known app called iOS simulator which I use from time to time when designing responsive sites. It’s a lot easier to get a site which is running locally as most things are in the really early stages of a project.
It makes it really quick for me to get something on a simulated phone and see how it will look. The alternative to this would be put the files on a server or use local tunnel or a similar service. However the time it would take me to figure out these solutions would take me away from designing. Which is what I want to be doing.
So I use a simulator to give me an idea of how the design works. It’s not an easy application to find as it’s wrapped up in xCode which you can get for free from apple.