If it's a normal looking coin being flipped in the air and caught before presentation, the only reasonable prior is that almost all of the probability mass shou

If it's a normal looking coin being flipped in the air and caught before presentation, the only reasonable prior is that almost all of the probability mass should be that it's completely fair. Maybe I'd leave 1% to spread amongst the rest just in case the adversary has some seriously devious tricks up their sleeve. Because physics - it's not possible to bias a rigid body so that it rotates with non-constant angular speed when flipped, as long as air resistance can be neglected, and that means that a fair flip gives 50/50 odds as to what side you catch it on. (Edit: clarity) If other stuff is going on, like you're letting it bounce or something, then it depends on the particulars. It's rather easy to load a die, for instance.