The Appraiser skill relates to trading and item value estimation. It is gained by trading, but has no associated assignable labor. Note that the skill will increase the first time the trading screen is opened for each caravan visit - a good way to train another dwarf is to assign him as broker, make him go to the trade depot and (T)rade, exit and assign the "main" broker again and make him do the actual trading.
In order to have access to the monetary fortress value estimates in the z-status menu, as well as individual item values, a dwarf with the Appraiser skill must be appointed as broker in the nobles screen. (Note that this is not necessary for the display of item quality - the indicators near the item names - which will always appear.)