There are many different methods of inspection, and the more that are undertaken and passed, the greater the confidence there is no problem. A significant crack may well produce an audible creak when flexed.
A bare fork suspended on a string should ring like a tuning fork when rapped lightly. A dull or buzzing sound could indicate an issue. For a valuable/difficult to replace fork, it may be worthwhile getting a magnaflux or magnaglow inspection at a qualified machine shop, before condemning. Cheapest, first step is throrough visual inspection after removal & cleaning. There may be some crack inspection dyes that can be purchased through retail outlets. Such dyes might expose a crack that is not otherwise visible.