You can look at the top card of your library whenever you want (with one restriction; see below), even if you don’t have priority. This action doesn’t use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.

If the top card of your library changes while you’re casting a spell, playing a land, or activating an ability, you can’t look at the new top card until you finish doing so. This means that if you cast a spell from the top of your library, you can’t look at the next one until you’re done paying for that spell.

You must pay all costs and follow all timing rules for spells cast from the top of your library this way.

If the top card of your library is a historic card with a disguise or morph ability, you can’t cast it face down from the top of your library with the permission granted by Crystal Skull because the resulting spell would not be a historic spell.

A card, spell, or permanent is historic if it has the legendary supertype, the artifact card type, or the Saga subtype. Having two of those qualities doesn’t make an object more historic than another or provide an additional bonus—an object either is historic or it isn’t.

Some abilities trigger “whenever you cast a historic spell.” Such an ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

An ability that triggers “whenever you cast a historic spell” doesn’t trigger if a historic card is put onto the battlefield without being cast.

Lands are never cast, so abilities that trigger “whenever you cast a historic spell” won’t trigger if you play a legendary land. They also won’t trigger if a card on the battlefield transforms into a card with the legendary supertype, the artifact card type, or the Saga subtype.