Smooth steering is good.
Is there any play/slop? Try to lock out your fork, and shake your handlebars back and forth, side to side (while keeping the bars straight). There should be no movement in the steer tube. If there is, it just may need to be tightened.. or, it could be a sign of something worse.
Why are you asking? It may be easier to describe your concern, other than ask for all the possible signs of a failing headset.