Cheating with foils and card orientation
The rules simply say that you cannot have marked cards, without enumerating all the specific ways they might be marked. From section 3.11 of the tournament rules:


  A card or sleeve is considered marked if it bears something that makes it possible to identify the card without seeing its face, including scratches, discoloration, and bends. 


So if someone is actually able to identify cards by messing with their orientation, or determining whether they're a foil, or anything else, they're breaking the rules. (Note that bends are explicitly mentioned, so that part of "is it a foil" is right out.)

Exactly how someone might get caught doing this and what the consequences are of course depends on the situation and the level of play. If someone accidentally has a card turned the wrong way at FNM, I doubt they'll be thrown out of the building. But if you're at a serious tournament and you notice your opponent carefully feeling the thickness of the top card of their library, things probably aren't going to go well for them.

In any case, the takeaway here for everyone is sleeve your cards and don't cheat. Cheating is bad, and if you think you see it, call a judge. But at an FNM or a kitchen table, it's possible to be friendly about little things. If there's a card out of place, just talk to your opponent, and fix things. They're probably a person who just like you is trying to play Magic and not cheat, and all it takes is a few words to turn the card around and get back to the game. I don't think any of this is a real problem - it's a kind of dumb way to try to cheat, one that won't ever get you much information - but if you do suspect it's happening, just pretend you're dealing with nicked sleeves and handle it however you'd handle that.