Appraisal is a technique in Slash'EM Extended. Every character can use it right from the start of the game. Technically, it is granted by the player character's race, but every race is set to have it.
Using this technique allows the player character to determine the enchantment value or amount of charges for their currently wielded item. Its main use is for weapons, thanks to the difficulty of determining their plus (although the weapon practice technique can do that sometimes, too), and upon appraising a weapon, the technique can't be used for about 200 turns. If it is used on something other than a weapon (e.g. a wand or magic marker), it will take about 2000 turns before the technique can be re-used.
The appraisal technique is a good way to determine the amount of charges left in wands or tools so the player doesn't run out of charges at a critical time, and it can also be used on a random non-BUC-tested ring (if it turns out to have a negative bonus, it's very likely to be cursed). However, the lengthy timeout means it cannot be re-used for quite some time, whereas using it on a weapon will yield a much shorter timeout, allowing the player to appraise lots of weapons to find the one with the highest enchantment. One downside of using it on weapons is that they may weld themself to the player's hands if cursed, so it would be a good idea to BUC test weapons on an altar first.
If the player wields an item that cannot have an enchantment value (e.g. a potion of healing, or the item's enchantment is already known), the technique will do nothing but still have a timeout. If the player is not wielding anything, though, the technique just doesn't work but can be used again immediately.
Since there is no penalty for using the appraisal technique, it is almost always a good idea to use it often.