When you have one card left, you must yell "UNO" (meaning one). Failure to do this results in you having to pick two cards from the DRAW pile. That is, of course if you get caught by the other players.

GOING OUT
A player who forgets to say UNO before his card touches the discard pile, but "catches" himself before any other player catches him, is safe and is not subject to the penalty. You may not catch a player for failure to say UNO until his second to last card touches the DISCARD pile. Also, you may not catch a player for failure to say UNO after the next player begins his turn. "Beginning a turn is defines as either drawing a card from the DRAW pile or drawing a card from your hand to play.
If the last card played in a hand is a Draw Two or Wild Draw Four card, the next player must draw the two or four cards. These cards are counted when points are totaled.

— both from http://www.wonkavator.com/uno/unorules.html

Letting players catch each other might be more complicated than it's worth; maybe the bot should always catch people who fail, or have a random chance (~75% maybe) to catch players who fail to say UNO.