What do you have to reveal to the opponent?

I had a somewhat unusual question that I had never considered.  Someone asked me if they have to reveal their battlegear's ability once it is flipped up.  This lead to a larger discussion about what needs to be revealed.  I had always assumed that stats could be always asked for(for attacks/ability requirements), but that abilities do not necessarily need to be revealed unless they are used.  For instance: My opponent flips destructozooka face up.  Do they need to reveal what element they picked there and then, or do they just note it when they perform an attack with that element.  Fundamentally, my question comes down to what actually needs to be revealed to the opponent at any given time(stats, battlegear/creature abilities, elements, etc) and what can be kept secret?
TL:DR  What information can the opponent ask for regarding your deck and cards?

Your opponent must be informed of flipped battlegear abilities, card text on locations, attack damage, mugic effects and card stats. basically if it is on the field or burst you need to tell your opponent about it

"My opponent flips destructozooka face up. Do they need to reveal what element they picked there"

Yes , then and there. If they choose right after drawing and playing an attack they could be lieing for advantage.

If you have face up effects or triggers you should always let your opponent know, or read them and should inform your opponent that they should do the same. It's just good practice and sportsmanship.