Transform your mobile device in a set-square with Right Angle app.
Through your camera, this app identifies a 90° angle between any two lines, creates a target and holds your screen.
All you need is position the mark over the object you want to identify the 90° angle.
There are some samples from this amazing app.
This app is available in Google Play here: https://play.google.com/store/apps/details?id=com.engcross.rightangle