I marked my BK 2 ways. One I used a silver sharpie (My keggle is painted black) to mark gallon increments on the outside. 2 I also used a pipe cutter and marked the handle of my SS mash paddle in 1 gallon increments to use as a dip stick. On my HLT I have a sight glass installed that I marked at half gallon incriments with a sharpie
I just took a piece of soft copper tubing and bent it so it would hang over the rim of my pot while nearly touching the bottom. I used a pair of side cutters to crimp it at each gallon. Works pretty well.
Edit, I also nearly cut/broke off the ends with the side cutter to close them off.
I also use a dipstick. Even calibrated it for thermal expansion, reading it at boiling measures room temp amount, ~1/4 gal difference @ 5 gal in my pot.