Wearing a wetsuit and carrying no heavy equipment (only a mask, snorkel, and fins), the weights are adjusted so that the diver floats vertically on the surface of the water, sinks when exhaling, and floats when inhaling.

Next, float to the surface with your heavy equipment on, and without kicking, deflate your BC.
If you sink at this point, you are overweight.

The proper amount of weight for neutral buoyancy on the surface is when you begin to sink after releasing the air from the BC and exhaling strongly.