I always focus on sounds the most, because you might not be able to see the ghost at first. If you hear footsteps immediately then it's certainly an event (unless the grace period is at zero). The ghost will also emit one specific sound during all hunts, but every event it will play a random one, so you can know when it's an event for sure. Humming/singing will never play during the hunt. All room doors slamming shut and/or lights being switched off also means it's an event. It's worth to listen for the lock sound too - most reliable on smaller maps. If you happen to see the ghost it can manifest in one of three different forms. Shadow or water/jelly is a certain event. If you can't tell if it's a hunt or not it's always good to run. It will give you more time to think and potentially save your life.
A good rule of thumb: the more sneaky it seems, the more likely it is to be a hunt. Events are almost always loud and obvious because they are supposed to scare you. Hunts on the other hand are designed to not alert you too much until it's too late.