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 lands played and spells cast from the top of your library this way.

If the top card of your library has {X} in its mana cost, you must choose a value of X that makes that spell's mana value 4 or greater in order to cast it.