What are the rules on Search Effects?

There are limitations to searching your deck, and as such players must know all the cards left in their deck just by what is on the field, in their graveyard, and in their hand. There are two types of search effects.

Mandatory Effects
Card effects that require you to look through your deck for cards have no real rules; If the effect Resolves, you must do it. However, if upon your search you find that no card exists that meets the search criteria, your opponent will have an opportunity to look through your deck and verify that there is nothing to search for. After they have finished, shuffle your deck.
 * Example: Sangan on the field is destroyed by Dark Hole and sent to the Graveyard. The owner of Sangan must search their deck for a monster with 1500 or less ATK. The owner cannot find a monster in their deck with 1500 or less ATK, so the opponent looks through it to verify.

Optional Effects
Card effects that you choose to activate will require you to know your deck inside and out. You are not allowed to activate a card effect to search for a card in your deck if you know that all the cards that meet the search criteria are no longer in your deck. To do so, then find there's nothing to search for and shuffle the deck is illegal, you are not allowed to shuffle your deck unless you are required to by a card effect. This is a form of stacking the deck and is considered cheating.
 * Example: A player controls a face-up Overdrive Teleporter. That player runs 3 copies of Psychic Commander and 2 copies of Mind Protector in their deck. That player searches their Graveyard, finding 2 Psychic Commanders and 1 Mind Protector. That player also has 1 Psychic Commander in their hand. Overdrive Teleporter's effect cannot be activated, because Overdrive Teleporter's effect Special Summons from your Deck 2 Level-3 Psychic-type Monsters, and the controller found 4 of their 5 existing outside of their deck.