Go to your local hardware store and get a 1/4" wooden dowel. Fill up the tanks and make sure they are leveled out and full. Dip your dowel in one of the tanks, scratch a mark at the point where the dowel is wet. Wipe the dowel off and wait till it's dry, use a permanent marker and make a ring around the dowel at the mark you made, this is your full mark or 120 Gal. Make another mark half way down, this is your half full or 60 Gal. Continue dividing up the dowel in as many sections as you want and write the gallons at each mark. I've been using this method for the past 15 years and it never fails, completely idiot proof and dirt cheap.
You can of course go the expensive way and have a tank level system installed, but it's a total waste of money in my opinion.