If the Measurement window is not open, press M to open it.
Click on the Angle button. Then click-and-drag in the photo. Drag the corner point of the two angle lines to the corner of where you want to measure the angle.

Release and drag the first end point of the first angle-line to the position where you want measure the angle from.
Repeat with the other end-point. If you want, you can give a name to the angle by entering it in the name field in the measurement window. Press Enter when you have entered the name to save it.

You can create any number of measurements.

If you print the photo, the angle measurement will be printed just below the photo.