If you’re in a practice space with no mirror, you can use your tablet’s front-facing camera as a mirror in a pinch. I did this just the other day. I popped my iPad on the music stand, opened the Camera app, and tapped the the camera rotation icon in the bottom right corner in order to switch the view to the front-facing camera.
You’ll pretty much only see your face, unless you stand way back. And unfortunately there is no zoom in/out for the image from the iPad’s front-facing camera (unless you use an app like Cam Mirror, which has zoom-in only and grainier image quality). But sometimes a view of your face is all you need – like yesterday, when I was watching my jaw while singing.
I suppose you could try this trick with your smartphone too, but I don’t know how well it will work with the small screen…