All of the fast (24++) rides I do have been going on long enough that everyone just knows. There are definitely stretches where the pace is relaxed and those where it's brutal. But again, everyone just knows. Well, everyone who's done the ride. First timers usually just get caught off guard and spit out the back... but get it next time.
Usually, if you stay in the front 1/3 of the group, you'll catch the acceleration soon enough to stay on. If you're in the back, you can easily miss it - because lots of those riders know about the fast sections but have not intention of keeping up.