Currently we do not off offer a ruler option, however, if you click grid lines and add a Horizontal or Vertical Grid Line.
When you select a grid line and look at the top left corner of the screen you will see where currently sits on your page. If you keep left click button pressed down and move it around, you will see the measurements change.
If place your mouse over the grid line so the 4 arrows, one going in each direction shows up, then you left click (keep holding) then right click, then let go of both (tricky I know) then when you can move your mouse around freely without holding a key down.