alas, since it is a classic brain teaser, it is much easier to answer, though no one likes the answer...
you can't.
you can count lines, but if a field is set to 'Can Grow' and it does, you have absolutely no idea that this has occurred.
all the other schemes are permutations on this idea, but since you don't if a line has grown you're out of luck.
the other permutation that I have tried is to assign heights each section and add them up. When they reach some value, you tell the report to move onto the next page...but I can see 'Keep Together' getting in the way as well since I don't think that CR tells you when it is going to skip the rest of the page...
Barring keep together and can grow, you can usually make a reasonable guess of where you are on the page and react as desired.
HTH