How do you know if your dog seen a ghost in your house?

I have heard that they will stand and bark into the direction and will not enter the room that they are occupying. I have also heard that the hair on their back and neck will raise much like when they are warning their owners of strangers. Thank goodness my dogs have never done this in my home.
My step dad died a couple of years ag and my mom swears her dogs sees him, she said they will stand and look up and bark and bark but she doesn't see what they are barking at. She says that they do not appear to be afraid or ready to defend.

Dogs sometimes display unexplainable behavior. I really doubt a ghost is at all ever a likely explanation.