The scroll, wand, and spellbook of light should all weigh less than other objects of those classes. This makes them trivially identifiable, but it isn’t considered game breaking to be able to tell these 3 objects.

Wands of light should light up when wielded (in a radius of perhaps 3 or 2). To give them some limiting factor, they either lose a charge each time they are wielded, or there is a fixed chance per turn it’s lit that it loses a charge. It’s been noted that this would provide an incentive to try wielding every wand in order to determine if it’s light, but this may not be a big problem.

The spell of light has an advanced form, which is a ray. It blinds monsters it is fired at like a camera flash does. (The camera flash is also transformed into a ray, which can be reflected.) In Wands Balance Patch-implementing variants, the wand of light should also have this effect for skilled users.

Izchak sells scrolls and wands of light, maybe spellbooks too, at low frequencies.