Zorua, you can request a re-roll/flip if the coin or die is slanted enough as if it was unclear on whether that result would have happened. The rule of thumb is you can re-roll if the die/coin landed at an angle of 40 degrees or higher.
Zorua, if you think the coin is weighted you can ask the other player to roll a (legal) die instead. Or when in doubt you can call a judge over and they can decide if the coin is okay or not.