Some zoom lenses dispense with DoF scales because estimating DoF for the zoom range is difficult, as opposed to a prime with a single scale. The Infrared marks are much easier to mark and include.
If you have a smart device you could bookmark a DoF calculator on the web, or find a DoF app.
If you are not so inclined, you could inscribe or sticker the DoF on the lens for various focal lengths.
EDIT: Thanks, google! Here's instructions on how to make one yourself:
And an image of a 35-70 DoF scale on a zoom lens: