Like sarbar, I use the GSI Soloist cup, if I happen to be using that cook kit. If I'm using my Jetboil, it has a similar cup.
If I'm carrying only a pot (typically titanium, or lately the Optimus Terra Solo), it will be a pot that has measuring marks stamped into it - if the amount of water I need is between two marks, I interpolate (well, OK, it's really just a guess, but "interpolate" makes it sound like I actually know what I'm doing.
)
In the past, when I used to use Nalgene bottles ("bless me, Platypus users, for I have sinned..."), I used the measuring marks on the side of bottle. Of course, that involved math: "Let's see, I've got 26 ounces of water in here, and I need to put 12 ounces in the pot...hmmm, 26 take away 12...if train A leaves the station..."