Use Spell Cards during your turn that allow yourself to know at least 1 card in your opponent's hand, so that this card's effect can be used during the same turn, such as "Dragged Down into the Grave" or "Exchange".
"Magical Contract Door" is especially good, since this card can be searched with it and ensure yourself knowing 1 card in your opponent's hand.
Good Trap Cards that let yourself know at least 1 card in your opponent's hand include: "The Forceful Checkpoint", "Mind Crush" (assuming your opponent does not have the maximum legal number of copies of the declared card in public locations after they discard), and "The Eye of Truth".
There is no need to force the opponent to reveal their hand; should they add a card directly from their Deck or Graveyard to their hand (through "Reinforcement of the Army" or "Mask Charge") and not use it immediately, declaring said card will ensure the effect of "Pluto" works. Also, some Decks have predictable strategies, making guessing what card they're withholding in their hand for a combo easy.