Here's a list of things often associated with hauntings
doors opening and closing
electrical turning on and off
faucets turning on and off
oddities on photos
oddities on video
feeling something touching you
feeling of being watched
feeling as if you walked through spider webs
objects moving on their own
feeling of someone sitting on your bed
feeling as if one was pushed
odd odors including cigars/flowers/pipe smoke
balls of light/orb seen by eyes
These were only some of the features, there are an unlimited amount of sensory input that tells us "ghost!"
It gets more complicated as any of these explanations can be the answer -
other dimensional beings
poor electrical connections
sound coming through ducts
open cleaning products
spirits of the dead
High EMF levels
environmental - open windows/moaning pipes
What do we have to perceive ghostly activity? We have our senses. Sometimes, we only hear them, but does that mean it was only a sound or that our eyes could not perceive the content - out of our visual range??? Conversely, we may see one, but not perceive the sound associated with it.
Our bodies have five fairly lame senses. We have the ability to perceive certain ranges, but others we cannot like infrasound or infrared. This might explain why some encounters are downright odd, including people seeing only part of an apparition or hearing only part of a sentence.
As we still have found no way to prove the existence of ghosts (in the concept we consider them - souls of the dead), it's impossible to prove if a place is haunted or not. We can certainly take every precaution to get rid of the explainable and get down to the unexplained and see what weight it carries. How much weight you put on personal experiences while investigating, psychic impressions or stories of those who witnessed it, is up to each investigator to determine. How much weight you put on EVPs that aren't Class A (language which listeners agree they hear the same words), is once again at your discretion.
To really understand a haunted place, going in for a few hours is not going to give an investigator a true insight. The people who live there are best to answer just how active it is and what they have experienced, but repeat investigations can provide amazing insights. I suggest more than two because either time could be a fluke - you want to find out what the trend is. As well, the place becomes familiar to where you have a better concept of what is around you, what is normal, what is not normal.
Ghostly activity can sometimes be playful, other times startling, and yet other times very frightening. It is mostly in the interpretation. Whatever you think a ghost is capable of, that determines how scared you become.
To most who live with them, ghosts are intermittent roommates who have their own focus, their own agenda, and only rarely notice or care you're there, just like how you feel toward them.