Where an App is a WebApp (versus a native app), it can be viewed live by clicking View within the Timeline.
It will appear in Studio, next to the Elements list in a mobile-first view.
In-Studio view of web Experiences
The Experience can also be viewed in a separate browser window via the right hand side button below the mobile view. Doing so would allow to stretch the dimensions to test responsive layouts.
Experience opened in a separate browser window
The Experience can also be viewed and shared via a QR code. Now every Event has a QR code - click on the QR icon below the mobile preview to download it, share it and scan it.
Event QR code