Interesting. I've gotten psychic visions before - more when I was a kid, but. They're difficult to pin down. Never used any tools though.
You can tell. They call it steam, it sort of - comes out of your mouth. It looks like fog. So you'd notice, and feel it. From what I can tell there aren't any people who feed off of steam near here but it's a big city so. If any show up I'll definitely let you know.