How do you know "how much is left in the keg"?

[william.roy.c]

I've never tried it myself, but I assume it gets you close. I actually tap the keg with my finger and listen to the sound. Kind of like thumping a watermelon. My method doesn't tell my how many pours I have left, but lets me know that I'm getting low.

I have used tally marks on a pad before. It gives you a chance to adjust for the size of the pour a little. Mixed success there.
Mostly, when I start to think that the keg might be getting low, I just see how much force it takes to tip it a little.