When a character uses a wand, roll one die. If the maximum result is rolled, the wand is half-used. If the maximum result is rolled for a half-used wand, it is now drained of power.The maximum result depends on the die used, of course. For wands, you'd probably want to use either a d20 or a d12. A staff traditionally has fewer charges, so a d10 or d8 may be in order. A rod or a cantrip wand would use a d6.
A drained wand can theoretically be recharged, or a caster could attempt to wring one last use out of the wand. Make a reaction roll: on a Very High result (12 on 2d6,) the wand works one last time; on a Very Low result (2 on 2d6,) the wand explodes for 3d6 damage.