To use the ruler
Click the dimensions pill found at the bottom left of the proofing screen.
Select use ruler, then use your mouse to click and drag to draw a line or box to see the measurements.
To change the units of measurements, click the units and select a different option (pixels, millimeters, inches etc)
Use the keyboard shortcut ‘-’ to turn the ruler on/off, or click off the proof.
You can change the scale.
To measure a diagonal line, draw a box around the element and you’ll see the diagonal measurement displayed along with the width x height.