I don't know, my coach has never hesitated to tell me when I'm fat.
But this isn't about me personally. this is about bodybuilding. when the play-by-play says "looks good, but seems to be holding water in his glutes," I can never see this "water"
When the "fat" is completely gone, you will know, your skin will be so thin it will feel dry or satin-like to the touch. The "holding water" look which is typically seen in places like the glutes, lower back, and lower abs is really just a thin layer of fat. In many instances this thin layer is hard to see under gym lighting, but becomes magnified under bright posing/stage lights. Same way "dark" skin in the gym can look washed out under stage lights.
You need to have the fat "gone" a few days out, then dehydrate enough to really thin the skin, then reload a little with carbs to keep fullness. Understand the nutritional connection between carbs and water.
Do not get sunburned or tan the day prior - this will thick the skin again. It's trail and error to some degree, but that is where the guru is supposed to come in.