I prefer the method of just putting some ice cubes in a glass. If they they still have their shape when I get back then they didn’t melt. And you can tell even if they just melted a little bit because they’re is a puddle at the bottom of the glass. Means I don’t need to think ahead and freeze a cup of water or find a penny.

Also, as I/mrlonely_ pointed out you could have a situation where most of the cup melts but the penny ends up on a little iceberg so it doesn’t drop.