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.

If the top card of your library has an Adventure, you can cast the Adventure spell this way.

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

If Johann leaves the battlefield and returns to the battlefield in the same turn, or if Johann leaves and another Johann appears, the casting permission granted by the old one is different than the casting permission granted by the new one. On the turn in which the new Johann enters the battlefield, you may use its casting permission once, even if you used the old Johann's casting permission to cast an instant or sorcery spell from the top of your library in that same turn. Note that if you control multiple Johanns you won't have the opportunity to cast an instant or sorcery spell from the top of your library before the "legend rule" gets you down to one Johann.