Well you are right in that when they do get sick it seems to happen fast, they are good at hiding illness until they physically can't. So it ends up seeming like they just got sick when in reality they have been sick for awhile. One of the best ways to monitor is keep track of weight, when sick they tend to stop eating, and therefore lose weight. Also watch the poop, that can indicate a problem as well if that changes size, frequency, or consistency.
The best way to avoid sickness is by giving clean, fresh water, good quality pellet only food, plenty of fresh hay, make sure the room is at a comfortably low temp with low humidity with no drafts, and there is no interaction between them and another animal species. Also supervise them when they are out to make sure they don't eat something they shouldn't, and that nothing they shouldn't eat or chew is in or near the cage.