You can read the label on tins. All player-made tins are unlabeled, while random tins will randomly (and deterministically) either say truly what sort of monster is in them, or have an irrelevant message. Messages could include: “Tuna Fish - expires Feb 6, 1989”, “Rare Meat of a Genocided [hallucinatory monster]”, “Water Chestnuts - be sure to eat while underwater”, “Yendorian Brand Applesauce”, “This Tin Is Not Booby-Trapped”, “Fresh Tinned Adventurer Meat”.