The only way I know of to check for sure is to measure your blood glucose and blood ketone levels. If your blood glucose is ~100 in a fasted state and your ketone levels are above 1 your undoubtedly a fat burner. However this method is rather expensive. (FYI, i have never felt the need to do this test on myself)

Here are some other ways to check: Lower your carb intake to ~20g per day, if you feel just as energetic, your a fat burner. Skip a meal, or two in a row. If you fell just as energetic, your a fat burner. Go for a LONG run or play session, if you never "Hit the wall" or "Bonk" or have a "second wind" your a fat burner.

From your use of the phrase "Fat burning beast" i guess you read Mark's Daily Apple. According to Mark, at 75g of carbs, you should be fat burning.

It probably varies from person to person, but here's how I know I'm in that sweet spot of running on fat:

(BUT...before I oversimplify, I should say that we are always running on both fat and carbohydrate, even on a lower carb plan. But most people here know what we mean when we say "fat adapted" or "running on fat" -- when the vast majority of your activity is fueled by fat - not just when you're exercising, but all your body's activity - breathing, blinking, sitting upright, cooking dinner, typing on PH...)

Like a previous poster said, you can tell for sure that you're burning fat if you test for ketones in your urine, but you can also be fat adapted and not show measurable ketones that way.

When I'm running well on fat:

I can go hours and hours without feeling hungry. And when I do feel hungry again, it's true, genuine hunger and not me feeling "hangry" - the combination of hungry & angry, where your blood sugar is low and you feel like you're gonna murder someone or possibly eat your own arm off if you don't get fed in 30 seconds, know what I mean? ;-) No irritability, no misdirected anger. And I don't have cravings for any junk when I get hungry. I honestly want real food - chicken, steak, fish, vegetables, cheese (I do dairy). I can walk past a cake, pan of brownies, or whatever, and not think twice about it. Not tempted in the slightest.

I'm bursting with energy both physically and mentally. My head is super-clear, very sharp thinking, and maybe this is just personal to me, but I tend to feel more even-keeled, optimistic, and ambitious when I'm on a good fat-burning jag. I am a major pessimist and it sometimes takes an act of Congress to get me to accomplish things, so when I'm feeling good and getting things done that's a sign, too.

Now if only I could get back there! Haven't been doing too well lately.. :(

You do not feel hungry when you are a fat burner - but it can take a long time to get there. You do not get moody, you do not get headaches. If you are a sugar burner those things happens with about 4 or 5 hours of nothing to eat.

I don't agree that "everyone is different". When it comes to glucose, everyone is the same in that your brain needs about 100-120g a day to function properly.

If you consume between 0 to 120g of carbohydrates, some of that energy requirement will be met by gluconeogenesis, and the rest from ketones.

If you consume 120g or more of carbohydrates, then your brain will selfishly use the first 120g, and any extra will be stored in your muscles and liver.

Since you are eating 120g of carbs, I very much doubt that you are in ketosis, so I doubt that you are "a fat burning beast", although of course everyone burns fat for aerobic exercise.