I have a keggle, and I use a plastic spoon/paddle. I took a box cutter, and sliced notches out of the spoon at each 1 gallon increment. Then, at the 6 and 7 gallon marks, I used a marker to make a ring all the way around the spoon at those notches so they can be seen easier. This just gives me a good feel that when I add my runoff to the kettle, then I am in the range of pre-boil liquid that I want to shoot for. If I am closer to the 7 gallon mark, then I boil a little longer. Just a gut check.
To be honest, if I am a half gallon off one way or the other, I don't sweat it. Typically, I prefer to be a half gallon high.....to have a half gallon more beer, but I usually end up the other way around.