There will be points in a duel where you will have to show your opponent your hand or deck. Why?. for confirmation; ie. to make sure you aren't cheating. Now many of you I'm sure would NEVER cheat, but that's not the point.
You will need to reveal you hand if your opponent has to declare a card name in you hand.
Type: Trap (Normal)
Text: Declare 1 card name; if that card is in your opponent's hand, they must discard all copies of it from their hand to the Graveyard, otherwise you discard 1 random card.
Your opponent or you can easily say you have none of the cards declared, which is why a confirmation is required. Check your hands for all copies and then tell them you no longer need to see.
You'll need to reveal you deck when you declare you have no targets in the deck to add or special summon. This will only be required for cards that are mandatory (does not include "can").
Text: When this card is sent from the field to the Graveyard: Add 1 monster with 1500 or less ATK from your Deck to your hand.
If you or your opponent declares that there is no monster searchable, the opposing side is required to check the opponent's deck for any targets for clarity.
Limits to Examining
While you search your opponent's hand/deck it is not fair for you to take notes of his /her cards.
You are not required to reveal your hand after you opponent "randomly selects a card" in your hand.
If I miss anything or if you have any questions or concerns, please do not hesitate to reply. I will attended to it as soon as I can.