For my brew kettle I made a dip stick by filling it 1/2 gallon at a time and using electrical tape to mark the increments. I used red electrical tape on a white plastic rod which is really easy to see, but the down side is that leaving it in the kettle for more than a few seconds would likely dissolve some of the adhesive from the tape into the wort.
An alternative method I have heard is to measure the depths with a ruler and derive an equation or make a table to convert depth of liquid to volume.