measure the hole in the bearing set it went through. Though I'm assuming you mean the diameter of the steering tube. The stem is the thing between the handlebars and the steering tube.

The steering tube is what goes through the head tube and into the fork. Either way, the above answer in post #2 should work.

Or do you have one of those old style '7' handlebar stems with a quill which might get more questions about what you are really asking for.