You can lay it on a hone (or anything you know to be flat). Gently putting it edge down will tell you about smiles/frowns, and laying it across (as in honing) on one side than the other will tell you about warping or uneven wear (or you could do flat strokes with magic marker on the bevel). I suppose you could also look/measure for uneven wear on the spine and/or bevel, but that would assume that the spine/bevel was even before it was put to a hone (which may or may not be the case).