Foils are slightly thicker than regular cards, more stiff and are usually a bit curved. Even with sleeves it’s possible to tell if a face down card is foil by looking at it or feeling/bending it a bit. (is there a legitimate situation where you could be given the chance to feel/bend a card that’s not revealed?)
Another thing is whether the card (sleeved or not) is pointing up or down while in the library. I always sleeve may cards and make sure they are all facing the same direction, but as far as I know it’s not required. It would be easy to turn one card upside down so I know when it’s on the top of my library.
It seems like these two things could be used to cheat. Are they any rules about this? Is it even an issue?
The rules simply say that you can not have marked cards, without enumerating all the specific way they might be marked. From section 3.1 of the tournament rules.
So if someone is acturally able to identify cards by messing with their orientation, or determining whether they are 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 depend on the situation and the level of play. If someone accidentally has a card turn 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 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 how ever you’d handle that.