You can't tell until you have a hygrometer. The real issue is change, or rather, rapid change, from high humidity to low. Because wood swells when it picks up moisture, the worst that usually happens with a guitar that has been in an unusually moist environment is that the belly will swell (like just after a spaghetti dinner) but later will return to normal. The same cannot be said of low humidity. A guitar that dries out below the humidity level at which it was built will crack. Plain and simple.
The worst is winter in the midwest or northeast - arctic air masses are low in humidity. So when the outside humidity is at 10% or so, watch out! The guitar is under incredible stress. The only way to combat that is with a humidifier and the exercise of some self discipline about checking it to be sure that you are in a range of about 40 % - 60% Relative Humidity (and stay there). The best way, however, is to leave it (case open) in a humidity controlled room. A small studio with a humidifier is the right recipe, but you need to know where that level is. So get a hygrometer.