If we aren't producing enough, the grid actually slows down, and it will speed up if we produce too much. Depending in where you live, the grid runs at 50 or 60Hz. Once it drops above or below a certain threshold, we know to produce more or less power. Basically the generators at power plants encounter more or less resistance if they aren't meeting demand perfectly, so they can spin faster or slower than they are supposed to.

We have also gotten very good at predicting people's power usage, so we are prepared for big spikes and drops that happen at regular intervals like people getting home from work. We also account for how much solar power we get and adjust our predicted demand based on that because you can't exactly just turn off solar panels