I have read that Sketch Mirror has the option to view your design in a desktop browser, tablet, & smartphone. However, for some reason I haven't been able to figure this out. Anybody have some advice?
Yes there is. Type ctrl+alt+i to trigger the developer tools. And click on the top second left icon to pick different screen size.
The handy shortcut: ctrl+shift+i, ctrl+shift+m
It also works for opera by the way.