How can you tell if there's a ghost in your home?

I get a feeling that when I'm home alone, playing with my dogs, I"m being watched. Like someone is behind me or upstairs...And one night when I came downstairs to get something I saw my dog just sitting on the floor, staring in the kitchen. He barked a few times and kept staring. It actually creeped me out. I then called his name to shut him up but he kept yapping. I called him again, he didn't do anything. I got closer and said his name and he looked at me and wagged his tail. And a few summers ago, I was outside and I quickly looked up and saw a face in the window...? It could of just been my imagination but, I can't be too sure.