Drop the wheel out and gently sqeeze the fork blades. An easy test to see if there is a crack in them.

Put the wheel in and Turn the wheel 90 degrees (or 45) to the frame. Apply the brake and gently rock the bicycle. This may give you a different result than with the wheel stright ahead.

It may be worth dropping the fork from the bike and visually inspect the steerer tube.

When you drop the fork out, be aware of the tightness of the lower cup.
A loose cup may indicate a problem with the head tube.