Test for Headset Adjustment
Your fork should be held securely in the bike's head tube, with no play whatever, but it should rotate freely without resistance.
Headset adjustment can be tested several ways. Do them all.
Holding the handlebars, pick up the bike, and rotate the bars. Do they rotate freely, with no binding? If not, you need to loosen the headset.
Holding the handlebars with the front wheel on the ground, squeeze the front brake, and rock the bike forward. Is there any looseness? Caution--the looseness could be in a suspension fork and not the headset. This test works best on a rigid fork. If there is looseness, the headset needs to be tightened.
With the bike in a work stand, grab the fork in one hand and the stem in the other, and try to move the fork to notice any flop. Is there any looseness? If so, the headset needs to be tightened.
Pick up the front of the bike 6 inches and drop it on the floor. Does the wheel bounce up with a nice, clear sound. When the wheel hits, is there a rattling sound? A rattle, or anything other than the bouncy-rubber sound, indicates a loose headset that needs tightening. Caution--the rattling might be your cables, so keep that in mind.