Click on the little backpack button on the bottom bar of your interface--the one that looks like the button for the Inventory screen. If the item is equipped (i.e. worn, held, or whatever) its ability will be there. Some items (wands, scrolls, and some others) go in your quick-item slots, in which case a picture of the item will appear in the bottom bar.
A quick note: once-per-day use items disappear from those selection options once they are used. They appear again after you have rested (and not after a full day, as the description would make you believe).