Tool for finding the center of objects

I am looking for a tool to find the center of bolts, and other types of objects.

I remember Scott Z. posting pictures of a small tool that he uses. I searched, and cannot seem to find it. It seemed small enough to fit in the vest pocket or tool pouch easily.

I have looked, and have seen center head combination squares, and they would fit the bill, but looked a little large for the average tool pouch.

I'm not in a hurry to purchase one, but may have another construction project coming up in the next year. On my last big hospital job, I had to a good bit of asbuilts on bolt locations.

Thanks in advance.:-)

Merry Christmas.

Jimmy