Freeing up fat is the goal of any diet.
If your body has burned enough of the fat that you have eaten and it has freed up the easily accessed stores (the fridge in Dr Fung’s analogy) then it starts burning the fat from your cells.
This is called ketosis. From the WebMD site (What Is Ketosis?).
Ketosis is a normal metabolic process, something your body does to keep working. When it doesn’t have enough carbohydrates from food for your cells to burn for energy, it burns fat instead. As part of this process, it makes ketones.
The article continues.
For healthy people who don’t have diabetes and aren’t pregnant, ketosis usually kicks in after 3 or 4 days of eating less than 50 grams of carbohydrates per day. That’s about 3 slices of bread, a cup of low-fat fruit yogurt, or two small bananas. You can start ketosis by fasting, too.
There are ways to detect ketones. The most accurate measurement is a blood test. Another measurement is easier and cheaper, a ketone test strip. The ketone test strip is activated by your urine.
I got a box (with a bottle in it) of 50 Relion Ketone Test Strips at Walmart in the pharmacy for about $7. That’s less then 15 cents a test.
Here’s a good article on using the sticks to measure your state of ketosis (An Overview of Ketone Testing Products).
For me personally, there’s another way. Taste in my mouth. If I am in ketosis my mouth tastes metallic. This site includes that along with other symptoms (What is Ketosis?).
a metallic taste in mouth
strong smelling urine
random bursts of happiness (it’s weird, but it’s true!)
If you are in doubt use the strips. I am in ketosis even after eating dinner (a LCHF dinner).
When the Strips stop working (Why You Need To Stop Worrying About The Color Of Your Ketostix).