Our cycle classes are 45 minutes long. Here's something I like to do at the 25 or 30-minute mark (by this time, we have done most of the drills at least once - flats, climbs, jumps, etc.)
I turn off all the lights in the room, and tell the members that they are on their own for 3 minutes, and that they can do any of the drills that we have done in class. If most of them do jumps and climbs, I know that they have some energy left to burn. If most of them choose a flat road, I can tell that they are tired.
The members enjoy having a few minutes to do what they want to do, and it helps me to decide whether to finish class with intensity, or to ease up.