Revealing a card means displaying it so the other players know what card it is. You can't just flash it and then hide it away again; if someone asks to read it, they are entitled to do so.
July 23, 2014 8:47 p.m.
I find the good rule of thumb is to leave a card which is normally hidden revealed until such time as the state of that hidden information changes.
So in the case of Auntie's Hovel I would leave the Goblin revealed until either all opponents acknowledged that they saw it, or until the next time the state of my hand changes (I draw a card, cast a spell etc)
July 23, 2014 8:57 p.m.
Some abilities make specific mention to how long a card is revealed for. For instance Forecast states that the card remains revealed until the end of the upkeep step or until it leaves your hand.
Different players play revealing cards differently. Many players will just leave the known portion of their hand face up on the table, see Feline Longmore, others will let their opponents see it then pick it up and shuffle it around. I prefer the former but it is entirely dependent on what you choose as long as your opponents have sufficient time to read the cards.