As others have said, this all depends on the judge. As an example of what can happen, I'll tell you about a recent case with me.
After initial shuffle in game one, two judges came and asked to do a deck check on both me and my opponent. They took our decks away, while we sat there awkwardly talking. Much later, they came back and said that both of our decks were officially "marked" because our sleeves were worn in a way where we could theoretically tell which card was which.
It took them a long time to do the deck check because a group of four judges were all trying to figure out if we had done so on purpose. To do that, they carefully looked at which cards were worn and if there is a pattern. Turns out, both me and my opponent were just playing with old sleeves. Some of the worn cards were basic lands, some where win conditions, some were just random non-special cards. We were allowed to play out our match, and had to resleeve our decks between matches. No big deal.
That example gets to the heart of what the rules really are: the judge has absolute power in determining what is and is not "marked." They have to because if the rules gave a list of the things that were considered cheating, a crafty cheater would just find a new way. But also notice that neither me nor my opponent got in any real trouble. The rules care about intent. If all of your win conditions are upside down, it's unlikely that happened by accident -- you are probably cheating. If one random land is upside down you probably just dropped a card and put it back into your deck wrong. If about 50% are upside down you probably flipped half when shuffling. Only one of those cases is grounds for getting in real trouble.
Since you asked specifically about foils, there have been cases where someone was playing with all-foil lands, and no-foil spells. I don't remember if they got a warning or a game loss, but they were told to swap out the lands with non-foil versions.