You can tell if cake batter is overmixed by observing certain signs:
Tough texture: Overmixing cake batter can develop excess gluten in the flour, resulting in a tougher and denser texture in the baked cake.
Air bubbles disappearing: When you overmix the batter, the air bubbles created during the mixing process can collapse. This can lead to a denser cake as the trapped air that contributes to a light and fluffy texture is lost.
Uneven texture: Overmixing can cause uneven distribution of ingredients, resulting in an inconsistent texture throughout the cake. You may notice pockets of dense or rubbery areas within the baked cake.
Sinking or sunken cake: Overmixing can cause the cake to rise rapidly in the oven and then collapse, resulting in a sunken or unevenly risen cake.
To avoid overmixing, it is recommended to mix the ingredients just until they are combined. It's better to have a few small lumps than to overmix and risk a dense and tough cake.