i foiled out my deck(dredge). bazaar is nonfoil; previously when the deck played power/LED, serenity, or undiscovered paradise they were also nonfoil, and 3 of my petrified fields are nonfoil(due to only being able to find one signed foil) when i play that card. contagion, when played in the board, is nonfoil.
the degree to which foils are marked can vary depending on which foiling process was used and how they've been stored; I flatted my foils in a press and store them in a box with silica packets so humidity is less of a concern. my deck is full of alters, so I always have to get my deck approved by a judge before any big event, and i've never had any problems come up with the cards being considered marked, but i've definitely seen foils that even double sleeved would be obviously visible as such. there was a particularly egregious promo yixlid jailer ended up on top of my opponent's deck one time when i didn't cut after he tinkered for citadel in a small event, though i don't know if it was intentional or not, the card was definitely marked.
I actually once had an opponent in a non-vintage event, in the process of saying that i might have marked cards, choose to demonstrate this concept by claiming that his own cards were marked and pull a foil and a nonfoil from his graveyard and say he could tell the difference by feel.
