Hold the front brake and gently rock the bicycle forwards and backwards, using your other hand to feel the junction between the headset cover and frame. Movement between those parts can indicate play. Brake components or a suspension fork can produce a similar knock, so locate the movement carefully.
On a threadless headset, the top cap adjusts bearing preload, while the stem’s side bolts clamp it to the steerer. Adjustment requires those side bolts to be loosened; afterwards, align the stem and secure it to the manufacturer’s torque specification. Check both play and free steering. Simply tightening the top bolt harder is no remedy.
If steering catches around the straight-ahead position, or the bearings remain rough after correct adjustment, wear may be responsible. More tightening cannot restore damaged surfaces. Have the assembly inspected in a workshop. Carbon forks and integrated cockpits require their specific manufacturer’s assembly instructions; threaded headsets use a different adjustment procedure.