The Eyes Have It by Philip Dick

It wasquite by accident I discovered this incredible invasion of Earth by lifeforms from another planet. As yet, I haven't done anything about it; I can't think of anything to do. I wrote to the Government, and they sent back a pamphlet on the repair and maintenance of frame houses. Anyhow, the whole thing is known; I'm not the first to discover it. Maybe it's even under control.
I was sitting in my easy-chair, idly turning the pages of a paperbacked book someone had left on the bus, when I came across the reference that first put me on the trail. For a moment I didn't respond. It took some time for the full import to sink in. After I'd comprehended, it seemed odd I hadn't noticed it right away.
The reference was clearly to a nonhuman species of incredible properties, not indigenous to Earth. A species, I hasten to point out, customarily masquerading as ordinary human beings. Their disguise, however, became transparent in the face of the following observations by the author. It was at once obvious the author knew everything. Knew everything - and was taking it in his stride. The line (and I tremble remembering it even now) read:... his eyes slowly roved about the room.