Whether you're evacuating because of fire or you're just leaving for a holiday, you'll want to know if the power went out while you were away for any substantial amount of time. Fill a cup with water, put it in the freezer until it's solid, then pop a coin on top. Leave it in there, and when you get back, check. You've probably figured this out already, but if the coin is on the bottom of a now-frozen cup of ice, the power went out — long enough for the ice to melt back to water in your freezer and the coin to fall through, and then the power came back and the water re-froze. If the coin is where you left it, the power stayed on, or only went out for short periods.
It's a fun trick, but it's practical too — if the power went out long enough for a cup of ice to melt — which takes a ridiculously long time in an insulated fridge or freezer — and then came back, your food in the fridge and freezer may be just as cold or frozen as you left them, even though they spent hours at room temperature or warmer. If you find a coin at the bottom, it might be a good indicator that you should clean out the fridge and not risk eating contaminated food.
It's worth keeping in mind though that ice floats in water, so if the power is out for enough time for some of it to melt but not all of it, there's no guarantee that you won't find the coin on top and think everything is fine — so when in doubt, throw it out. However, even with a short outage, fridges and freezers insulate well enough to keep most food items fresh. But if that coin is on the bottom, you have problems. It's a simple tip, but a helpful one if you're going to be away from home.